wm: Add ActivityMetricsLaunchObserverRegistry

Public(@hide)-visibility app sequence launch events.

Anything in system_server can use this new API to subscribe to
the ActivityMetricsLaunchObserver from the ActivityTaskManagerInternal
LocalService.

Also change AMLO to expose ActivityRecordProto as a byte[]; the previous
parameter was ActivityRecord, which is package-private.

Also add systrace for measuring performance of the AMLO callbacks
(seems to be about ~250us on a marlin-userdebug; mostly because of the
protobuf serialization).

Test: atest WmTests:ActivityMetricsLaunchObserverTests
Bug: 72170747
Bug: 112680320
Change-Id: Icc07b727b50438bdd65bb01978e15d839809948b