libbinder_ndk: use __ANDROID_VENDOR__

This library is used by the product partition, which also defines
__ANDROID_VNDK__. Previously, since the only relevant library here was
the gsi service, we excluded APEXes from this test. However, this leaves
regular product binaries able to talk to vendor over unstable APIs.

Bug: 180646847
Test: binderStabilityTest (is in TH)
Change-Id: I896e0bc8aec3c35f43df2d5fdd03ac92fcc84c30