Use `mContext.getMainLooper()` as the parameter for creating a new
Handler if it's null in `registerCallback()`

Test: refactoring CL only, all tests should pass
Bug: 293436997
Change-Id: I2a0d4735c3324a7c2e8ae9ef432350613562148f
diff --git a/media/java/android/media/projection/MediaProjection.java b/media/java/android/media/projection/MediaProjection.java
index 223b432c..4059291 100644
--- a/media/java/android/media/projection/MediaProjection.java
+++ b/media/java/android/media/projection/MediaProjection.java
@@ -109,7 +109,7 @@
         try {
             final Callback c = Objects.requireNonNull(callback);
             if (handler == null) {
-                handler = new Handler();
+                handler = new Handler(mContext.getMainLooper());
             }
             mCallbacks.put(c, new CallbackRecord(c, handler));
         } catch (NullPointerException e) {