Simplify shim content validation logic.

We're going to extend the shim apex to contain also the CtsShim*
packages. The existing logic is cumbersome to extend, so this CL
simplifies it by simply specifying the accepted files in a constant on
top of the file.

Test: atest --test_mapping system/apex
Bug: 138429615
Change-Id: I5d4fec8c94c4401c0d5769dd623bf1eecd7d0a5a
1 file changed