Handle CancellationException in BackupTransportClient

Logic added in ag/16968183 can produce a CancellationException which is
not currently caught, resulting in system server crashes (see attached
bug). Catch the exception and update tests accordingly.

Bug: 224777563
Test: atest BackupTransportClientTest
Merged-In: Ibee991dbc9673c7b6b4d0051f09b4e8862ad6af4
Change-Id: Ibee991dbc9673c7b6b4d0051f09b4e8862ad6af4
(cherry picked from commit d0c42dc6)