Tests and tidy up for the zonetrees code

Some logic has been tidied up, methods renamed
and responsibilities moved to enable easier
testing.

The tests revealed a bug that caused the first
time zone transition after 1970 to be skipped so
this has been fixed and the tzlookup.xml file
regenerated with update-tzdata.py.

Bug: 72142943
Test: ran all tests, see tzlookup_generator/README.android
Test: ran update-tzdata.py and looked at tzlookup.xml
Change-Id: Ia344cee9fe19628d798c055d01c776a5f8e0e272