Only update cache for new user on user create

This change optimizes our handling of user additions to only compute the
cache for the newly added user and its interactions with existing users.
Prior to this change, we'd recompute the entire cache on user add.

Test: atest AppsFilterTest AppEnumerationTests
Fixes: 187853334
Change-Id: I09cb9f98b34d86ae678373d3f61dd5fdd334cfb7
Merged-In: I09cb9f98b34d86ae678373d3f61dd5fdd334cfb7