PinnerService: Use the default UidObserver implementation.

A class which overrides IUidObserver.Stub with empty callback
implementations was added in change
I2ff1e868586861e4dcd6586ad22139ba84eaf39c to simplify BroadcastQueue
interface complexity. Using this default implementation will mean less
churn when new callbacks are added to IUidObserver, or existing callback
method signatures are changed.

These callbacks do not need a "throws RemoteException" modifier, so that
has been removed, as these can't be marked as override because the
UidObserver class doesn't have them.

Bug: 274486653
Test: Presubmit, smoke test on cuttlefish.
Change-Id: I39d5da65a3e5eff6cea00709dacee3d4b3c6075d