Add CtsShimPrivUpgrade to shim.v2_apk_in_apex_upgrades

The v2_apk_in_apex_upgrades APEX shim will be useful for testing fixes
for apk-in-apex caching bug.

Test: m com.android.apex.cts.shim.v2_apk_in_apex_upgrades
Bug: 225435110
Change-Id: I5d2270b8341c0405bf256c417acff4279c6b8754
Merged-In: I5d2270b8341c0405bf256c417acff4279c6b8754
(cherry picked from commit c6c98bdf)