servicemanager: getUpdatableNames()

This new method is a reverse of updatableViaApex(). It returns the list
of declared instances which can be updated via the passed APEX.

Updatable vendor apexes are supposed to be used only to update HAL
services. APEXd can use this method to see if the target APEX is
actually to updating HALs.

It's not exposed to NDK/Java yet because there's no clients.

Bug: 254201177
Test: TBD
Change-Id: I7b5aa7d00a3ddeb13855816006a9561dfa601529