add bt-in-background preference

And clear it from a button in the alert that's posted when the
foreground service notification is selected.

xwords4/android/app/src/main/java/org/eehouse/android/xw4/BTService.java

@@ -351,7 +351,9 @@ public class BTService extends XWService {
         Log.d( TAG, "startService(%s)", intent );
 
         if ( ! sInForeground && canRunForegroundService() ) {
-            context.startForegroundService( intent );
+            if ( XWPrefs.getBTBackgroundEnabled( context ) ) {
+                context.startForegroundService( intent );
+            }
         } else if ( sInForeground || Build.VERSION.SDK_INT < Build.VERSION_CODES.O ) {
             context.startService( intent );
         }

xwords4/android/app/src/main/java/org/eehouse/android/xw4/DlgDelegate.java

@@ -57,6 +57,7 @@ public class DlgDelegate {
         SET_HIDE_NEWGAME_BUTTONS,
         DWNLD_LOC_DICT,
         NEW_GAME_DFLT_NAME,
+        DISABLE_BT_BACK,
 
         // BoardDelegate
         UNDO_LAST_ACTION,

xwords4/android/app/src/main/java/org/eehouse/android/xw4/GamesListDelegate.java

@@ -1360,6 +1360,10 @@ public class GamesListDelegate extends ListDelegateBase
                                      true );
             break;
 
+        case DISABLE_BT_BACK:
+            XWPrefs.setBTBackgroundEnabled( m_activity, false );
+            break;
+
         default:
             handled = super.onPosButton( action, params );
         }
@@ -2271,9 +2275,11 @@ public class GamesListDelegate extends ListDelegateBase
     private void tryBackgroundIntent( Intent intent )
     {
         if ( intent.getBooleanExtra( BACKGROUND_EXTRA, false ) ) {
+            ActionPair pair = new ActionPair( Action.DISABLE_BT_BACK,
+                                              R.string.disable_bt_back );
             makeNotAgainBuilder( R.string.not_again_btservice,
-                                 R.string.key_notagain_btservice,
+                                 R.string.key_notagain_btservice )
-                                 Action.OPEN_GAME )
+                .setActionPair( pair )
                 .show();
         }
     }

xwords4/android/app/src/main/java/org/eehouse/android/xw4/XWPrefs.java

@@ -52,6 +52,16 @@ public class XWPrefs {
         setPrefsBoolean( context, R.string.key_enable_sms, enabled );
     }
 
+    public static void setBTBackgroundEnabled( Context context, boolean enabled )
+    {
+        setPrefsBoolean( context, R.string.key_btback_enabled, enabled );
+    }
+
+    public static boolean getBTBackgroundEnabled( Context context )
+    {
+        return getPrefsBoolean( context, R.string.key_btback_enabled, true );
+    }
+
     public static boolean getDebugEnabled( Context context )
     {
         return getPrefsBoolean( context, R.string.key_enable_debug, false );

xwords4/android/app/src/main/res/values/common_rsrc.xml

@@ -61,6 +61,7 @@
   <string name="key_notify_sound">key_notify_sound</string>
   <string name="key_disable_relay">key_disable_relay</string>
   <string name="key_notify_vibrate">key_notify_vibrate</string>
+  <string name="key_btback_enabled">key_btback_enabled</string>
   <string name="key_enable_sms">key_enable_sms</string>
   <string name="key_enable_p2p">key_enable_p2p</string>
   <string name="key_network_behavior">key_network_behavior</string>

xwords4/android/app/src/main/res/values/strings.xml

@@ -2026,6 +2026,9 @@
     <string name="title_enable_p2p">Enable WiFi Direct</string>
     <string name="summary_enable_p2p">Experimental, uses lots of battery</string>
 
+    <string name="title_btback_enabled">Enable background Bluetooth</string>
+    <string name="summary_btback_enabled">Always listen for Bluetooth messages</string>
+
     <!--  -->
     <string name="confirm_sms_title">Confirm your SMS plan</string>
     <!--  -->
@@ -2649,6 +2652,8 @@
   Bluetooth moves and invitations while in the foreground, but any
   sent while it\'s in the background will be lost.</string>
 
+  <string name="disable_bt_back">Disable Background</string>
+
   <string name="no_invites">This game has sent no invitations</string>
 
   <string name="disable_dualpane">Disable side-by-side</string>

xwords4/android/app/src/main/res/xml/xwprefs.xml

@@ -335,6 +335,11 @@
                         android:summary="@string/notify_other_summary"
                         android:defaultValue="false"
                         />
+    <CheckBoxPreference android:key="@string/key_btback_enabled"
+                        android:title="@string/title_btback_enabled"
+                        android:summary="@string/summary_btback_enabled"
+                        android:defaultValue="true"
+                        />
     <CheckBoxPreference android:key="@string/key_enable_p2p"
                         android:title="@string/title_enable_p2p"
                         android:summary="@string/summary_enable_p2p"