[Android MR] Fix a crasher when registering/unregistering callback

The crasher happens when the session is connected/disconnected. The code
registers/unregisters RemoteMediaClient callback upon
connecting/disconnecting. However it should check the availability of
RemoteMediaClient of the current session.

Bug: 711860
Change-Id: Ie4b29f507a0044ac8649b52b37f1541e4dc9b4ee
Reviewed-on: https://chromium-review.googlesource.com/c/1350211
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611030}
2 files changed