Add new api ClipDescription.getTimestamp.

ClipDescription.getTimestamp can be used to know the time
at which the associated ClipData is copied to global clipboard.

Bug: 30873580
Test: cts-tradefed run singleCommand cts-dev --module CtsContentTestCases -t \
      android.content.cts.ClipDescriptionTest
Change-Id: I9486ab3fe0696959d42f6b4c98e40f5becee3019