codec2: fix issue in allocating too many 8k buffers

default output delay is too large, in 8k use case, framework doesn't
allow to use so much memory to allocate those 8k buffers.

if exceeds max capacity and underlying buffer alloc fail. do not
create MediaCodecBuffer holds a null underlying ABuffer. Otherwise,
some MediaCodecBuffer APIs may access ABuffer (nullptr).

Bug: b/208481412
Change-Id: Ic8e33f6c27ee1c435bf0dc4bc09df87cb0ad3c3a