Allocate buffers before calling start on its source in OMXCodec::start() for encoder case

o This makes it possible to configure the source to use the same number of input buffers
  as requested by the video encoder, before the source starts. As a result, hardcoded
  number of video buffers for camera source, for instance, can be avoided.

o related-to-bug: 6920805

Change-Id: I13d2c308dce34967768cd407f02988e92ef10a89