Check if surfacecontrol is valid in getBufferTransformHint

Fixes a crash when app queries the buffer transform hint but
VIMPL's surfacecontrol is not valid. We should fail gracefully
by providing a default since its hard for the caller to always
know when the surfacecontrol is valid.

Fixes: 301201605
Test: presubmit
Change-Id: I27ce4608bb0de5cbb41afd78d9cd1eb40729e96a