MediaAppender+MediaMuxer:AppendFunction, new APIs

Added test cases to test appending data to MPEG4 files and two new
APIs, GetTrackFormat and GetTrackCount in NDK MediaMuxer.
MPEG4 test files with only audio, only video and one audio and one
video are added as inputs to the tests.
New formats can be added to the test just by adding suitable test files.

Bug: 154734325

Test: atest android.mediav2.cts.MuxerTest
      -- all the new tests are passing.
Change-Id: I60063f6f25e08d67d8dcffe64a58ec3b0c43968c
4 files changed