add filter command in exported_flag_check

This change add filter-api-flags command to this tool. It should be
triggered when the build system creates the exported flags jar of all
flags for API. It will remove all flags exported but not for APIs from
the final jar.

Test: atest exported-flag-check-test
Bug: 345798636
Change-Id: I3af0b915a6f3e6ba07d41a5f24ecac7f7f8f37a6