Make 'IHalAdapterVendorExtension' unstable again

There is no benefit in having this interface as "stable" because
it still does not allow to use 'AServiceManager_isDeclared' for it.
Keeping it as "stable" requires updating all the manifest fragments
of its implementations.

Flag: EXEMPT bugfix
Bug: 378948881
Test: m
Change-Id: I040e7bce41d21fa2a63280a7c218c6b68240edcd