Add system exemption for power restrictions

* Introduce the appop
  OP_SYSTEM_EXEMPT_FROM_POWER_RESTRICTIONS.
* An app with this appop will be made exempt from all
  power restrictions, including app standby and doze.
* In addition, the app will be able to start fgs from
  the bg, and the user will not be able to stop fgs
  run by the app.

* This new appop is replacing an appop in Android U that was unused.

Bug: 246330879
Test: atest AppOpsTests
Change-Id: I9ecdb529f4342e676d2a9587637e9efdd59059d6