ActivityManager: 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.

Also rename ActivityManager.UidObserver to MyUidObserver to remove any
ambiguity.

Bug: 274486653
Test: Presubmit, smoke test on cuttlefish.
Change-Id: I6e30a4e116a7baaca1e411ca0cc4a4dca37a32bd