Fix boot jar handling when both source and prebuilt APEXes and modules

are present.

1) The boot jar to APEX mapping is maintained by the base names for
both of them. When building with prebuilt modules and APEXes, that
means we need to take care to compare them without regard to any
"prebuilt_" prefixes.

2) VisitAllModules can visit disabled modules and both source and
prebuilt modules, so they need some conditions to skip modules that
aren't applicable for boot jars.

Test: `m droid`
Test: `m droid SOONG_CONFIG_art_module_source_build=false`
  with fresh ART Module prebuilts in place
Bug: 171061220
Change-Id: Iced269d29127bc8b8f9b3171adb60a97d115628b