mediandk: clean up AMediaCodec_getInput/OutputBuffer semantics

AMediaCodec_getInputBuffer erroneously considered the
offset, which could result in a smaller input buffer being
returned and client confusion. In practice, input
buffer offset is rarely used.

Similarly, the buffer size returned from
AMediaCodec_getOutputBuffer included padding after the
output buffer leading to potential confusion.
Now the size returned from both AMediaCodec_getOutputBuffer
and AMediaCodec_dequeueOutputBuffer is correct.

Also removed mentions of non-existing NDK methods
AMediaCodec_getOutputBuffers and AMediaCodec_getInputBuffers.

Bug: 301470262
Flag: EXEMPT bugfix
Change-Id: I6bbd9b85023aef56a608362afde0662d3df7284a