Add empty check to RemotePlayback app ID

"Invalid Application ID" crashes have surfaced now that the new
RemotePlayback pipeline is turned on by default in M72. The custom app
ID fetched from the AndroidManifest should never be empty in theory, but
in practice it seems like it can be.

This CL fixes the issue by adding an empty check, and falling back to
the default media receiver if ever we aren't able to fetch a valid
app ID.

Bug: 902871
Change-Id: I573f9a11433c1394cee99459a3154539be8d7081
Reviewed-on: https://chromium-review.googlesource.com/c/1354570Reviewed-by: Zhiqiang Zhang <zqzhang@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612027}

chrome/android/java/src/org/chromium/chrome/browser/media/router/cast/remoting/RemotingMediaSource.java

@@ -106,7 +106,7 @@ public class RemotingMediaSource implements MediaSource {
                 // Should never happen, implies a corrupt AndroidManifest.
             }
 
-            sApplicationId = (customAppId != null)
+            sApplicationId = (customAppId != null && !customAppId.isEmpty())
                     ? customAppId
                     : CastMediaControlIntent.DEFAULT_MEDIA_RECEIVER_APPLICATION_ID;
         }