libbinder: Consider 'VNDK + APEX' as system stable

Currently, we have 'use_vendor' on APEX which means that
'__ANDROID_VNDK__' things might not be logically coupled with the vendor
partition. Since APEXes can't be used from the vendor partition now, we
can mark 'vndk && apex' things as having a stability corresponding to
the system partition.

Bug: 136027762
Test: boot && binderStabilityTest
Change-Id: Ib784c0da6bbb4435aec0f8c1fd1bf421332d64d7