Use Calendar.Builder for MAP Client timestamp parsing

MAP Client was reporting different timestamps for a given
message-listing object each time getUnreadMessages() was called. The
milliseconds value was always different. This was a result of the
Calendar object pre-populating all fields with the current date-time. The
reported date-time would always have the current time's milliseconds
field.

Datetime parsing has been updated to use a Calendar.Builder which is
clearer and doesn't use the current time when initializing. The
milliseconds field is explicitly set to zero.

Tests were added for the ObexTime class to prevent this from happening
in the future.

Bug: b/135606822
Test: atest ObexTimeTest.java
Change-Id: Ie872b4bb30f68e0c00b15767eb0413a6e67ba630