Export boot image files from prebuilt_apex/apex_set

Previously, the prebuilt art-bootclasspath-fragment did not provide any
boot image files. That meant it was impossible to build any module that
requires access to those files from prebuilts, e.g. any module that
needs to be dexpreopt-ed.

This change enables that module to retrieve those files from the
prebuilt_apex.

Bug: 177892522
Bug: 189298093
Test: m nothing
      m droid SOONG_CONFIG_art_module_source_build=false SKIP_BOOT_JARS_CHECK=true
      - the previous command does not work but this change does fix one
        of the issues reported.
Change-Id: I1d4d9545172d79282918130df6b9aa55471bffc1

java/dexpreopt_bootjars.go

@@ -358,6 +358,19 @@ func (image bootImageConfig) moduleFiles(ctx android.PathContext, dir android.Ou
 	return ret
 }
 
+// apexVariants returns a list of all *bootImageVariant that could be included in an apex.
+func (image *bootImageConfig) apexVariants() []*bootImageVariant {
+	variants := []*bootImageVariant{}
+	for _, variant := range image.variants {
+		// We also generate boot images for host (for testing), but we don't need those in the apex.
+		// TODO(b/177892522) - consider changing this to check Os.OsClass = android.Device
+		if variant.target.Os == android.Android {
+			variants = append(variants, variant)
+		}
+	}
+	return variants
+}
+
 // Return boot image locations (as a list of symbolic paths).
 //
 // The image "location" is a symbolic path that, with multiarchitecture support, doesn't really
@@ -489,7 +502,7 @@ func copyBootJarsToPredefinedLocations(ctx android.ModuleContext, srcBootDexJars
 	}
 }
 
-// buildBootImage takes a bootImageConfig, creates rules to build it, and returns the image.
+// buildBootImage takes a bootImageConfig, and creates rules to build it.
 func buildBootImage(ctx android.ModuleContext, image *bootImageConfig, profile android.WritablePath) {
 	var zipFiles android.Paths
 	for _, variant := range image.variants {