Add @HalfFloat annotation for fp16 data types stored in shorts

This CL has a companion CL to add the @HalfFloat annotation to the
support library.

Test: cts-tradefed run singleCommand cts-dev --module CtsUtilTestCases --test android.util.cts.HalfTest
Bug: 29940137
Change-Id: I4e1dc456687c1c026437150e9cc94a54f3264d4e