sdm: Clear old content metadata when gralloc buffer metadata is dropped

HWC layer metadata extraction currently relies on gralloc flags
indicating what metadata fields have been set. Update content
and timestamp metadata extraction to handle the case where a new
buffer has a different metadata state. When a metadata field is
dropped from the buffer, the corressponding cached metadata in
the layer is dropped as well.

Change-Id: If4bc4fae5208de5eba9c3825a5f1a6230ccf65dc
1 file changed