Don't require merge_annotations_dir for annotations_enabled

As far as I can tell, annotations_enabled does not require use of
merge_annotations_dir (although it may have done in the past).

Bug: 157010342
Test: m
Change-Id: I6705522774203b4566f58679bbc2f0c17422c1dd
Merged-In: I6705522774203b4566f58679bbc2f0c17422c1dd
(cherry picked from commit c5e13279)