Camera: Correct gainmap format check

Use the 'inputFrame' gainmap format when
checking for a valid entry.
Currently the check is done within 'processInputFrame'
which doesn't hold the internal 'mMutex'. In this
scenario 'mGainmapFormat' can change independently
from the 'inputFrame' and the gainmap muxer track
initialization can still be skipped.

Flag: NONE bugfix
Bug: 444177086
Test: Camera CTS

Change-Id: I9edd0669b009eeb3e7904d2aa2718acabb2000cb