Add truncate_timestamp annotation

Mark privacy-sensitive atoms with truncate_timestamp annotation.

Factor out annotation collation to a helper method.

Add truncate_timestamp annotation support in stats-log-api-gen.

Add writeAnnotation* calls in Java and native generated code for
atom id annotations.

TODO: remove kTruncatingTimestampAtoms from atoms_info.
TODO: use truncate_timestamp annotation inside statsd.

Bug: 151111680
Test: stats-log-api-gen-test
Test: m statslog-framework-java-gen
Test: m libstatsmetadata

Change-Id: I3db5f4ffbf959bd36c62f890cc88606912798d40