Make apex_available respect AllowMissingDependencies

When SOONG_ALLOW_MISSING_DEPENDENCIES is defined in the build, don't
fail if an apex_available entry is not available. This is for smaller
Android manifests that may not include all APEX projects (e.g. master-art).

Bug: 142300164
Test: able to build from master-art manifest.
Change-Id: Ice1274f7dd143c4b1bf756e2cde93e217266affa