Add dex_import module type

Add a module type for importing a prebuilt jar that contains
classes.dex files.

Test: m with a prebuilt jar that contains classes.dex files in
      PRODUCT_BOOT_JARS
Test: java_test.go, dexpreopt_test.go, dexpreopt_bootjars_test.go
Bug: 124804356
Bug: 125517186

Change-Id: I496848f9dca11f758d49b1cb68168cec7f8e1718