Keep CtsShimPriv's v2/v3 signature

The build system silently destroys APK's v2/v3 signature, even with the
PRESIGNED one.  This change prevents that.

Test: apksigner shows v2/v3 signature are reserved (after manual copy to
      the prebuilts)
Bug: 133505716
Change-Id: I34bab87157400b36bdf6a62f3c692523fcf2911c