Add Flag for Metrics Deduplication Effort

For our recent Metrics work, we have been working to solve duplicated
rows. This flag aims to capture bugfixes to prevent duplicated rows from
being emitted in the first place.

Bug: 362994633
Bug: 362995084
Test: Flag Test
Flag: android.credentials.flags.fix_metric_duplication_emits
Change-Id: I18381a2df55f323c031b83de64f202e49654be00