Add new module-lib APIs for MTS testing

Several android.timezone classes have already been exposed for the
telephony module work, so these tests provide coverage for those.
Additional APIs have been exposed for MTS testing, i.e. to provide
greater confidence that the tzdata module data is correct / is
being read correctly.

Also, small changes to existing code to make them consistent with new
classes. Small docs improvements.

Bug: 147884233
Test: see system/timezone change
Change-Id: I047b29f17a41993f859947a6d6c3685896fe4cb6