Add new POWER_SAVER permission & update api

This CL adds a new privileged permission called POWER_SAVER that
will allow whitelisted packages to toggle battery saver on the
device. This can be done via PowerManager, where the API for
setting battery saver has been updated to accept calls from apps
with either DEVICE_POWER or the POWER_SAVER permission.
Additionally, we whitelist Turbo for the permission.

Test: Framework builds, Turbo can toggle EBS
Bug: 115524274
Change-Id: I49d9747b2d42f792a2f3ba90a15aa23c47e489b3