RELAND MediaCodec: don't cache buffers until requested

Do not cache input/output buffers until explicitly requested.

Track validity of buffers before getInput/OutputBuffers so that the
dequeued buffers are marked accessible when the array is returned
to the client.

Bug: 189889230
Bug: 236106096
Test: manual
Change-Id: I6892f02853f00c3e355c827ec943fbdf183469f1