Added runtime flag for forced-app-standby

Added a global setting which can be configured to get metrics on the
battery savings from this feature.

Test: atest \
FrameworksServicesTests:\
com.android.server.job.BackgroundRestrictionsTest

Fixes: 70579515
Change-Id: I541f2aac39847f9bf372240cafc4e0f0e5ebdc86