change MainActivity's launchMode to "standard"

reverse change in 19fa3fce05 (April 4
2019) making MainActivity singleTask, which caused starting the app to
launch the gamesList screen rather than the game that had been open when
it was moved to background. Now it's "standard," and I have to hope that
doesn't produce new problems. As I wrote back then, we'll see. More
quickly I hope.

xwords4/android/app/src/main/AndroidManifest.xml

@@ -55,7 +55,7 @@
 
     <activity android:name="MainActivity"
               android:label="@string/app_name"
-              android:launchMode="singleTask"
+              android:launchMode="standard"
               >
       <intent-filter>
         <action android:name="android.intent.action.MAIN" />