Stop recording unsupported class loaders.

Don't record anything if any of the classloaders in the chain are
unsupported.  (This is slightly unsubtle, but matchs the existing
behavior - see below.) We can't correctly DexOpt secondary dex files
in this case anyway.

Also discard any existing records of unsupported classloaders on read.

Currently this is almost a no-op: BaseDexClassLoader doesn't notify if
there are any unknown class loaders, since
5ac512c07cfa80160e240c359349c1390a20a981, so the new check is
reachable only if there's some sort of mismatch. But I'm going to
change that soon.

Tightened up visibility a bit too.

Bug: 111336847
Test: atest --test-mapping services/core/java/com/android/server/pm/dex
Change-Id: I6af5620a73be7b6440cbafdf6a5c1da1082cbdd4