Move deprecate_fsv_sig flag to another package

The previous flag isn't picked up by the build system and is missing in
/system/etc/aconfig_flags.textproto. To get included, it currently
requires an explicit dependency, e.g. from services.jar.

To avoid an ad-hoc workaround in an upper level Android.bp for this
flag, switch the feature flag to another package which is already in
frameworks/base/AconfigFlags.bp.

Bug: 277916185
Test: saw the flag in /system/etc/aconfig_flags.textproto
Test: use the flag
Change-Id: I6e9ffbd1356703fcba9972ba9dd1cb9886d52d84