Provide app launch count in UsageStats

This counts the number of times the app was launched from outside
the app and ignores intra-app activity transitions.

Introduce a new permission for registering to observe app usage.

Fixes a bug where Settings couldn't force the app into another
bucket if it was recently launched.

Bug: 74335821
Fixes: 76100712
Test: Manual test using Settings
Test: UsageStatsTest to verify permission change
Change-Id: Ibd343c1cfa37089a3ac6fc30ba3194e21a9be499