move backstage_power_flags_lib to framework flag list

This change makes backstage_power_flags_lib visible to metalava so
metalava won't hide the guarded api.

Since the guarded api are in the framework, this change move this flag
to framework flag list.

Bug: 333995906
Test: lunch sdk-next-eng && m sdk dist and presubmit
Change-Id: Id21ae3029a2201a879e53248de2a5c5e6a969ec7