Revert "Revert "Expand @FlaggedApi(FLAG) constants in API signature files""

This reverts commit 157d0c18.

Reason for revert: revert with fix to java_defaults instead of filegroup

Test: m checkapi
Test: m <partition>/etc/aconfig_flags.textproto & diff against a clean
      build
Change-Id: I757c6e87d81768ef6095a4bea67c74c3ae6028a7