SF: Unittest Coverage for HDR/per-frame-metadata

Cover the new additions to use the ComposerHal getPerFrameMetadataKeys()
function to determine what type of HDR metadata is supported.

This eliminates a warning in the tests about an unexpected call to the
function.

Test: atest libsurfaceflinger_unittest
Bug: 74827900
Change-Id: I4d0106116e1d45ba1b72612a9ba8634c36738ddf