Additional refactoring of MediaPlayer tests.

Rename MediaPlayerTestHelpers to MediaPlayerTestBase, a base class that can
be extended to easily create new MediaPlayer tests.  MediaPlayerTest now
consists of tests of local files, and MediaPlayerStreamingTest consists of
tests of streams (such as the local HTTP streams that were previously in
MediaPlayerTest).

Change-Id: I9aac07e8fdbc79bcd763e0a5e0c16254c42b39b0