Clarify forced reason usage.

Forced was meant to mean "forced by the user." Renaming the constant and
clarifying the comment so it doesn't get used incorrectly. Also
introducing a "forced by the system" constant and fixing the code so the
correct reason is used.

Bug: 145551233
Test: atest AppIdleHistoryTests
Test: atest AppStandbyControllerTests
Test: atest UsageStatsTest
Change-Id: I0d258d83141670728b758a3ef0056b19fde4a88b