binder: consistent vendor #defines

There is mixed use of __ANDROID_VNDK__
and __ANDROID_VENDOR__, so make sure they
are set consistently.

This is only possible because the VNDK is
deprecated. Before this, __ANDROID_VNDK__
may be set without __ANDROID_VENDOR__ (and I
think some of the logic in libbinder was
probably wrong at that point).

Bug: N/A
Test: N/A
Change-Id: I9cf106ab8309520229ae9e23e7d5c42ab3e425cf