am: Add new LaunchObserver to track app launch sequences

Add interface for code outside of activity-manager to get callbacks when an application
launch sequence occurs.

This will be connected in a follow-up CL to iorapd (see also IIorap.aidl) to enable
faster application start via prefetching.

Test: manual + logcat
Bug: 72170747
Bug: 112680320
Change-Id: I87abde9fa3f739abc5db4ccec48d2068ea32980d