Create new battery saver mode

Creates the concept of a "dynamic" battery saver mode controlled by
an external input in the framework. This mode behaves similarly to the
current automatic percentage based battery saver minus the fact that
the trigger is not static and that we might want to disable it at
a different level than when it was triggered.

Test: atest BatterySaverStateMachineTest
Bug: 111450127
Change-Id: Iac0de4f8f0336ed8870d2397574bd8885b7aa6a2