libbinder_ndk: warn on AIBinder_markVintfStability

This API should simply not be called by someone in Android - it's called
by the AIDL compiler, and it is not exposed to apps (it is only for
platform developers).

Bug: N/A
Test: N/A
Change-Id: Ib2c597e6bdcf125781fecd34cdd490e8e5c4e195