Initialize CameraX on a background thread.

We saw a situation where CameraX initialization could lock up when
trying to obtain system resource. We already fallback to the
Camera1Fragment when CameraX isn't initialized, so it should be safe to
do this initialization on its own thread.

src/org/thoughtcrime/securesms/ApplicationContext.java

@@ -386,11 +386,13 @@ public class ApplicationContext extends MultiDexApplication implements DefaultLi
   @SuppressLint("RestrictedApi")
   private void initializeCameraX() {
     if (Build.VERSION.SDK_INT >= 21) {
+      new Thread(() -> {
         try {
           CameraX.init(this, Camera2AppConfig.create(this));
         } catch (Throwable t) {
           Log.w(TAG, "Failed to initialize CameraX.");
         }
+      }, "signal-camerax-initialization").start();
     }
   }