Treat "apex" system_ext the same as platform

Change https://r.android.com/1672245 added support to treat
system_ext:foo in the boot jars configuration the same as platform:foo,
at least for dexpreopt_bootjars.go's getBootJar.

This change replicates that mechanism in platform_bootclasspath and as
that now handles hidden API processing also made a similar change in
the isModuleInConfiguredList function.

Bug: 177892522
Bug: 154976937
Test: m nothing
Change-Id: I105f4fbaa3b0355b013b7c5618d218d888faefb6