[Media Projection] Fix LAST_ACTIVE_SESSION having the wrong timestamp

It can happen that stop is called before the session was initiated,
when apps wrongly reuse the token.

The fix here is to only register the last active session, when there
actually was an active session ongoing.

Test: atest FrameworksServicesTests:MediaProjectionMetricsLoggerTest
Fixes: 307223500
Change-Id: I1dc5a08fd6d7e0d9cc42c7c7ce7b33284309b7c5