Improve MediaProvider WW atoms

Rename MediaProvider atoms to past tense verbs.

Slightly redefine the atoms for MediaProvider event metrics.
The changes are:
1. Remove timestamps from all atoms (because WW already attaches
timestamps to its events).
2. Log UIDs instead of package names.

Test: build
Bug: 149669087
Change-Id: I6bfb1da7e645f86d1b4a850f68614bc8fe105568