IMediaSource: add check before calling readMultiple

IMediaSource::readMultiple() fails when called for large
buffers which cannot be handled inline over binder's
reply data.
Add canReadMultiple() call to get value from native
implementation where Source can check buffer size and
decide if readMultiple is required to improve read latency.

CRs-Fixed: 1029437
Change-Id: If1f77898809c7ecc1ff07b37da1faa74b1c51c64