GSI: install flattened cts shim

The flattened version of com.android.apex.cts.shim.v1 should be
explicitly installed because the shim apex is prebuilt one and
PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES is not supported for
prebuilt_apex modules yet.

Bug: 158169437
Bug: 159392784
Test: m com.android.apex.cts.shim.v1_with_prebuilts.flattened
      # should install the prebuilts of shim APKs
Change-Id: I8b37a320d2130d7b33aca856bc067dce37d1fb30