Adding selective filtering to LTZP events

This commit introduces a time threshold for passing on "equivalent"
events for location time zone providers. When TimeZoneProviderEvent
instances are created, they are assigned a time from the elapsed
realtime clock, which will enable comparison with other types of events
generated by other flows. Events that have a sufficiently different time
will be passed to the system server. The time will be in the order of a
few minutes, the intention being to filter out high frequency events
that would cause the system server to do work unnecessarily, while
keeping a coarse idea of how recently the LTZP reported the state.

This commit adds the time to the TimeZoneProviderEvent and plumbs
through the configuration for the filter threshold from the system
server to the client code. The threshold can be set with server flags,
which are mostly intended to support the test logic.

Changes made at the same time:
1) Fix an issue in TimeZoneProviderRequest.equals() with an incorrect
reference equality check.
2) Rename some long-winded constant names - LOCATION_TIME_ZONE_PROVIDER_
-> LTZP_
3) Change some server flag strings so that "ltzp_" is spelt correctly
(which I do not anticipate using, but does mean that S and >S devices
respond to different flag strings).

Bug: 197624972
Bug: 200710190
Test: atest core/tests/coretests/src/android/service/timezone/
Test: atest services/tests/servicestests/src/com/android/server/timezonedetector/location/
Test: See associated cts/ change
Change-Id: Id682d420be21b9215b92f54bad599517c322fb6a