Revert the time zone display name logic

This cl is a partial revert of ag/1490275, and only
reverts the method getTimeZoneOffsetAndName. Reverting
it because it fails the PTCRB test.

After this revert, it will consistently show the zone
long name in Date & time page (which is the behaviour
in N).

Bug: 62077986
Test: SettingsLibTests
Change-Id: I8174bd7a3de09e476bbffefa6e71d997340dc7d8