Fix Date.toString() behavior / tidy up comments

Fix Date behavior / tidy up TimeZone comments and tests.
This commit contains a mix of documentation, test and non-test changes
to address issues found when testing the use of zic 2019a for producing
Android's tzdata file.

Comments marked with http://b/73719425 can be removed once we can be
sure of the version of zic Android uses (in a later release).

Bug: 73719425
Bug: 129926912
Test: CTS: run cts-dev -m CtsLibcoreTestCases
(cherry picked from commit 93d2da378c92c09b6e2e2401b3acef4c3d1118a5)

Change-Id: I3d9293399cea5b57d860bf062a57ff61ae52592a