Contents validation of override_apex's bootclasspath fragments

https://r.android.com/3083813 added support for overriding
min_sdk_version, and creating separate variants of transitive
dependencies of override apexes. This causes issues when there is a skew
between apex boot jars of source and prebuilt mainline modules. For
mediaprovider google apex, we now have _two_ source variants of
mediaprovider bcp fragment module.

This CL uses ApexInfo.BaseApexName to skip the check on both variants of
bootclasspath/systemserverclasspath fragments.

This CL also removes the prefix based check to determine if prebuilts
are active. If RELEASE_APEX_CONTRIBUTIONS for a specific mainline module
is set, it will be assumed that prebuilts for that mainline module are
active.

Test: go test ./apex ./java
Bug: 361771739

Change-Id: I01f7ef437df8f10b74ffa397ba461ff4f1f9bed6