Fix CallDetailActivity tests.

+ Reset the AsyncTaskExecutor's instance between calls in tests.
I don't know why this is necessary, but it fixes a class of problems
which were happening.

+ Don't try to release a media player if it has not been prepared.

+ Handle possible race conditions since MediaPlayer's async prepare
may be buffering or finished when a test assert is executed.

+ Add asset file no longer provided by variablespeed library.

- Cleanup some stream copy code.

Change-Id: I0ae5fde00514c6dcdb1e9c063435a13eed6e8528