Android 11.0 compatibility (#195)

* Update dependencies and target SDK
* Update Gradle to 6.6.1
* Update target API level to 30
* Update AGP to 4.0.1
* Update Android Build Dependencies (androidx.appcompat:appcompat:1.2.0, androidx.multidex:multidex:2.0.1)
* Update readme
* Move /sdcard/lovegame directory. (closes #194)
* Potentially fixes black screen when reopening LOVE from app drawer (fixes #190)
* Downloader cleanup.
* Merge SDL AndroidManifest.xml to LOVE
* Implement partial "Share to LOVE" support.
* Enable resizeableActivity

love/src/jni/SDL2/android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -243,6 +243,8 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
 
     // love2d-mod-start: allow restarting of the native thread
     public void startNative() {
+        boolean hadSDLThread = SDLActivity.mSDLThread != null;
+
         // Set up JNI
         SDL.setupJNI();
 
@@ -283,6 +285,10 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
                 SDLActivity.onNativeDropFile(filename);
             }
         }
+
+        if (hadSDLThread) {
+            resumeNativeThread();
+        }
     }
     // love2d-mod-end: allow restarting of the native thread