Export dex implementation jars from prebuilt_apex

Dexpreopt and boot jars package check all require access to dex
implementation jars created for java_library and java_sdk_library. They
were available when building from source but not when building from
prebuilts, even though they are embedded within the .apex files that
are referenced from prebuilt_apex.

This changes adds support to prebuilt_apex to export the dex
implementation jars and updates java_import to use those exported dex
implementation jars.

In a source build dexpreopt/boot jars package check access the apex (or
platform) specific variant of a java_library, e.g. core-oj, from which
it retrieves the dex implementation jar path.

After this change in a prebuilt build dexpreopt/boot jars package check
behave in the same way except in this case they retrieve the dex
implementation jar path from the apex (or platform) specific variant of
the java_import, e.g. core-oj.

The work to export files from a `.apex` file for use by other modules
is performed by a new `deapexer` module type. It is not used directly
in an `Android.bp` file but instead is created implicitly by
`prebuilt_apex`,

In order to do that this contains the following changes:
* Adds a new `dexapexer` module type to handle the exporting of files
  from the `.apex` file.
* Adds an exported_java_libs property to prebuilt_apex to specify the
  set of libraries whose dex implementation jars need exporting.
* Creates apex specific variants of the libraries listed in the
  exported_java_libs property.
* Adds the set of exported files to the ApexInfo to make them available
  to the apex specific variants.
* Prevents the prebuilt_apex variants from being merged together as
  they will not be compatible.
* Modifies java_import to use the exported file for variants of a
  prebuilt_apex.
* Adds a ninja rule to unpack (using deapexer) the contents of the
  prebuilt_apex's apex file, verify that the required files are present
  and make them available as outputs for other rules to use.
* Some minor refactorings to support these changes.
* Adds tests to cover prebuilt only, prebuilt with source preferred,
  and prebuilt preferred with source.

Test: m nothing
Bug: 171061220
Change-Id: Ic9bed81fb65b92f0d59f64c0bce168a9ed44cfac
diff --git a/java/dexpreopt_bootjars.go b/java/dexpreopt_bootjars.go
index 062005b..80211fd 100644
--- a/java/dexpreopt_bootjars.go
+++ b/java/dexpreopt_bootjars.go
@@ -422,16 +422,19 @@
 // Note that the same jar may occur in multiple modules.
 // This logic is tested in the apex package to avoid import cycle apex <-> java.
 func getBootImageJar(ctx android.SingletonContext, image *bootImageConfig, module android.Module) (int, android.Path) {
-	// Ignore any module that is not listed in the boot image configuration.
 	name := ctx.ModuleName(module)
+
+	// Strip a prebuilt_ prefix so that this can access the dex jar from a prebuilt module.
+	name = android.RemoveOptionalPrebuiltPrefix(name)
+
+	// Ignore any module that is not listed in the boot image configuration.
 	index := image.modules.IndexOfJar(name)
 	if index == -1 {
 		return -1, nil
 	}
 
-	// It is an error if a module configured in the boot image does not support
-	// accessing the dex jar. This is safe because every module that has the same
-	// name has to have the same module type.
+	// It is an error if a module configured in the boot image does not support accessing the dex jar.
+	// This is safe because every module that has the same name has to have the same module type.
 	jar, hasJar := module.(interface{ DexJarBuildPath() android.Path })
 	if !hasJar {
 		ctx.Errorf("module %q configured in boot image %q does not support accessing dex jar", module, image.name)