Document new preview callback methods more clearly.
Document new preview callback methods more clearly.

bug:2358064
Change-Id: If6af1fbf81b78b2a7ad98ce2c398145fd33200ce

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base