Update java.util.Date to OpenJDK8u60

Added two missing Android-changed comments.
Added upstream logic for converting to/from Instant
Added tests for conversion to/from Instant.

The minimum millisecond time (just under a second after
new Date(Long.MIN_VALUE)) that can be converted from
Instant to Date without encountering ArithmeticException
is not clearly documented. A test now ensures that this
boundary value remains consistent; no attempt was made
to change or explain the current boundary value.

Test: CtsLibcoreTestCases
Bug: 29935305

Change-Id: I79aff68ace5157cd3db5b1304651d07999d66796