Fix mmap size 0 errors

ACodecBufferChannel was attempting to create a MemoryDealer
with zero size, which resulting in mmap failures.  This was
causing the ExoPlayer GTS test to fail to complete due to
the Drm HAL being blocked in createPlugin.

Test: run gts -m GtsExoPlayerTestCases

bug:36870961
Change-Id: I0945e95565ea4ef05e53fb13cf5ddbba43f27173