Skip the platform availability check for unbundled builds.

Necessary to be able to use the standard Mainline module targets
aosp_${ARCH} on unbundled branches.

Also clean up a superfluous ")" that got added to the
not_installable_file target.

Test: m TARGET_PRODUCT=aosp_arm64 com.android.art
Bug: 172480615
Change-Id: Id85d0ffc8fe90c3e861666e6e2db963e0d9468a2