Set apex_available of libbinder_headers

libbinder_headers was available to the APEXes via the hand-written
allowlist in build/soong/apex/apex.go. Trying to remove the allowlist by
adding apex_available property to libbinder_headers.

Bug: 150999716
Test: m
Change-Id: I9f1c6ec20d1c665ff148b3b78b7e01eebde5a229