LowPowerStandby: Add APIs to get/set custom Policy

The Low Power Standby Policy declares:
- Packages (by package name) that should be exempt from
  Low Power Standby restrictions
- Reasons why an app may be dynamically exempt from Low Power Standby
  restrictions.
- Features apps are allowed to use during Low Power Standby.
  The features are Strings, so that vendors can define custom features
  for functionality that may not always be available or allowed in
  all Low Power Standby policies.

The existing exemption for active Voice Interaction Sessions is migrated
to use a predefined allow-reason, and enabled in the default policy.
When a policy without the VIS allow-reason is set, then apps having an
active VIS are no longer exempted from Low Power Standby restrictions.

Bug: 234002812
Test: atest LowPowerStandbyControllerTest LowPowerStandbyTest
Change-Id: I8747ae5b49613122e922aae7a917cbd2771ae091