servicemanager: add tests for updatable-via-apex

This is a follow-up of 76944fee.
With an example vendor apex installed on a cuttlefish device, the added
tests test two new APIs regarding updatable-via-apex.

- ServiceManager.updatableViaApex(instance)
- ServiceManager.getUpdatableNames(apexName)

This also fixes both methods
- (minor) updatableViaApex returns early when the match found
- getUpdatableNames iterates over all instances to find updatable
  instances.

Bug: 254201177
Test: atest servicemanager_test
Change-Id: I165c5532b40a4f9d520bef55864708b1629e24c7