Use APK CtsShim prebuilts for flattened apex.

Fix: 158036322
Test: built flame-userdebug with m OVERRIDE_TARGET_FLATTEN_APEX=true, ran
atest CtsAppSecurityHostTestCases:android.appsecurity.cts.PrivilegedUpdateTests#testDisableUpdatedSystemApp
and verified that the cts shim apex is not installed

Change-Id: Icc52500cfc0a555e11d1a467b2da0649c7f5d31f
Merged-In: Icc52500cfc0a555e11d1a467b2da0649c7f5d31f
(cherry picked from commit c9a786e5f77a5c06da6c07f718e4a7619a75d9e6)