Tidy-ups and refactoring

Extracted changes to reduce the size of upcoming changes. These do not
alter behavior.

The biggest change is associated with async execution of logic in the
time_detector. The time_zone_detector will need something similar soon
and this new approach is easier to test. Changing the time_detector will
ensure consistency.

Other changes are renames of parameters, variables and javadoc
improvements.

Bug: 222295093
Test: atest services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorStrategyImplTest.java
Change-Id: Ibedeafa10f7d73bbdaf6055073c7cf1a79026e83