Fix memory leaks

These are `new`ed, but never passed to anything else. It looks like
simply stack allocating will do what we want here.

Caught by our static analyzer:

frameworks/av/media/libmediaplayer2/MediaPlayer2AudioOutput.cpp:524:5:
warning: Potential memory leak [clang-analyzer-cplusplus.NewDeleteLeaks]
frameworks/av/media/libmediaplayer2/MediaPlayer2AudioOutput.cpp:527:9:
warning: Potential memory leak [clang-analyzer-cplusplus.NewDeleteLeaks]
frameworks/av/media/libmediaplayer2/MediaPlayer2AudioOutput.cpp:529:9:
warning: Potential memory leak [clang-analyzer-cplusplus.NewDeleteLeaks]

Bug: None
Test: Ran the analyer again. It was happy.
Change-Id: I8f4cda4a5993f43ad50cf5a0d152ed7158d7c8e1