NetworkPolicyManager : Process low power mode change on background thread

Since PowerManagerService runs in the same process, the callbacks occur on
the invocation thread. updateRulesForGlobalChangeLocked can take quite long
to process which in turns blocks the system processes background thread queue
and delays things like switching to/from power saver mode.

Address this by processing the action on our background thread via the handler,
and to not queue up multiple calls.

Change-Id: Ibbcae795f0138a1e1836f0df92f905178d34b684