Move CtsShim and CtsPrivShim APKs out of system.

These packages are now available in the shim apex package, which is made
available in both flattened and unflattened apex configurations.

Bug: 138429615
Test: atest CtsPrivilegedUpdateTests CtsPermissionTestCases
CtsStagedInstallHostTestCases; (on both flattened and unflattened
configurations)
Exempt-From-Owner-Approval: approved in
https://android-review.googlesource.com/c/platform/build/+/1180943
Change-Id: I03939f6c3e711d5887da201e32173f86a6b0289e