Move Time coretests to their own test module

Move Time coretests to their own test module to handle
http://b/305192485 (inability to land TEST_MAPPING changes because
FrameworksCoreTests is perma-broken).

Some javadocs and other non-functional changes have been made to test
source to keep checkstyle happy.

Bug: 305192485
Bug: 182461754
Test: Treehugger
Test: atest FrameworksTimeCoreTests
Change-Id: Ia4ab69c8971c828d5bbe8c646382b5d0b03d58e5