libbinder: allow stability downgrade

For instance, use a VINTF interface inside the system image without
adding it to the VINTF manifest (e.g. for testing, in this case, the
interface can't be used between system and vendor).

Bug: 183154648
Test: binderStabilityTest
Change-Id: I38019e226547b2437b920e79c252c7e55c17f7e4