[android] enable image shared element transition for API 21+22

There's an Android framework bug (#224270) on these APIs that causes a NPE
when the shared element is not visible anymore when returning.
Since we know restore the list position, the shared element should be
visible and thus not produce NPEs anymore.

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationActivity.java

@@ -853,15 +853,11 @@ public class ConversationActivity extends BriarActivity
 		i.putExtra(ATTACHMENT_POSITION, attachments.indexOf(item));
 		i.putExtra(NAME, name);
 		i.putExtra(DATE, messageItem.getTime());
-		if (SDK_INT >= 23) {
-			String transitionName = item.getTransitionName();
-			ActivityOptionsCompat options =
-					makeSceneTransitionAnimation(this, view, transitionName);
-			ActivityCompat.startActivity(this, i, options.toBundle());
-		} else {
-			// work-around for android bug #224270
-			startActivity(i);
-		}
+		// restoring list position should not trigger android bug #224270
+		String transitionName = item.getTransitionName();
+		ActivityOptionsCompat options =
+				makeSceneTransitionAnimation(this, view, transitionName);
+		ActivityCompat.startActivity(this, i, options.toBundle());
 	}
 
 	@DatabaseExecutor