media: fix race condition on async release

Clear Java object state at releaseAsync(), and delete native object
upon async release complete.
Clearing CodecBase object only led to a rare race condition which
caused null pointer dereference.

Bug: 158501286
Test: atest CtsMediaTestCases:MediaCodecTest#testAsyncRelease (100 times)
Test: atest CtsMediaTestCases:MediaCodecCapabilitiesTest#testGetMaxSupportedInstances
Test: atest CtsMediaTestCases -- --module-arg CtsMediaTestCases:size:small
Change-Id: I691d39007c0ea770318f4038558ad338252bd2fb