Normalize timestamps in host java libraries

Remove timestamps from jar files in host java libraries to get
consistent results between builds to help with build artifact
caching.

Bug: 38215808
Test: sha1sum desugar.jar && touch Desugar.java && m -j desugar && sha1sum desugar.jar
Change-Id: I1ecac9b2b80f673937086686540ab0736259df1b