media: keep JMediaCodec::mCodec reference until destruction

Releasing mCodec prematurely at release() sometimes causes double
free on MediaCodec objects.

Bug: 134770912
Test: atest CtsMediaTestCases:MediaCodecTest#testAsyncRelease (20 times)
Change-Id: Ieb265c9c4d74bd6bd3b34f0a713c8fa7dd2dedd1