Use apex name in apex_manifest.pb as the mount point

When bind-mounting flattened APEX, use the apex name found in
apex_manifest.pb as the mount point, instead of the directory name which
might be different from apex name in case when the apex is overridden.

This allowed us to remove the special casing for the ART apex since we
/system/apex/com.android.art.release will be mounted to
/apex/com.android.art instead of /apex/com.android.art.release.

Bug: N/A
Test: m
Test: OVERRIDE_TARGET_FLATTEN_APEX=true m, device is bootable
Change-Id: Ibdde7002b9078db390e6672b0eb82c474925451d