Fix default value for mUidState.get.

In updateRulesForWhitelistedPowerSaveUL, we are using
mUidState.get(uid). So if the uid didn't already exist, it will
give 0 which represents PROCESS_STATE_PERSISTENT and this is
inconsistent with it's other usages.

Bug: 35766310
Test: verified that apps coming to foreground, while the battery saver is on,
      have access to network.
Change-Id: I42a8455862e3a15e14825d9cf74e05a6f1dbe298