Chanage uid to userId where it denotes an Android user id.

This patch changes the variable name `uid` to `userId` where it
denotes an Android user id. This makes it harder to mistake the user id
for an actual UID.

Test: Trivial refactoring no logical changes.
Bug: 163636858
Change-Id: I9e240367bcd30bbfa1816014a7f44bbcb58ede4b