libbinder_ndk: Add stability downgrade

This can be used to allow registering a service with a VINTF interface
without requiring a manifest declaration.

This is useful for test services or in the case of a system proxy
service that implements the same interface as the vintf service.

Bug: 183154648
Bug: 177664629
Test: atest binderStabilityTest
Change-Id: Ibd8dad46945a339b77dca70484082fda06220ee3