stagefright: Remove additional deep-copy of encoder buffers

MPEG4Writer makes a copy of encoded buffer to avoid holding on
to the read buffer. This is not needed with MediaCodecSource,
as the encoded-output buffer is already copied to a heap buffer.
This saves a copy and some power.

However, cloning the buffer is still needed for upstream sources
that cannot afford to keep the buffers with writer up until they
are released.
So, pass a hint in buffer's metadata to indicate if it is OK
to delay the release and not copy the buffer.

Change-Id: I9661dafa4d45ff87364f5dfb1b2f7d01552a83fe