add a bool resource matching BuildConfig.DEBUG

I want to be able to have some boolean prefs default based on whether
it's a DEBUG build.

xwords4/android/app/build.gradle

@@ -180,9 +180,11 @@ android {
             minifyEnabled false // PENDING
             // proguard crashes when I do this (the optimize part)
             // proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+            resValue "bool", "DEBUG", "false"
         }
         debug {
             debuggable true
+            resValue "bool", "DEBUG", "true"
             minifyEnabled true // for testing
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
             // This doesn't work on marshmallow: duplicate permission error

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

@@ -72,6 +72,9 @@ public class XWApp extends Application
         Assert.assertTrue( s_context == s_context.getApplicationContext() );
         super.onCreate();
 
+        // Remove me soon!!!
+        Assert.assertTrue( BuildConfig.DEBUG == getResources().getBoolean(R.bool.DEBUG) );
+
         ProcessLifecycleOwner.get().getLifecycle().addObserver(this);
 
         android.util.Log.i( TAG, "onCreate(); git_rev="