Package dexpreopt artifacts for libcore jars in the ART apex.

This patch adds dexpreopt files for the libore part of the
bootclasspath to the ART apex.

Since this is specific to the ART apex and makes not sense for other
apexes, the patch does not add a new module property, but only a
boolean flag denoting that this is an ART apex.

Dexpreopt artifacts packaged into the ART apex differ from those that
are packaged in the system image: it inludes only the libcore part of
bootclasspath jars, but not the framework part. When the boot image
extension is implementd, dexpreopt artifacts for the libcore jars will
be removed from the system image (but for now they are both in the
apex and in the system image).

Build rules for the new set of dexpreopt artifacts are created using
a new variant of the global boot image config. Previously we had two
variants: "default" (for the system image) and "apex" (for the
JIT-zygote experiment). This patch adds the third "libcore" variant.

Test: m
Test: m com.android.art deapexer \
    && find $ANDROID_BUILD_TOP -type f -name 'com.android.art.*.apex \
        | xargs deapexer | grep boot \
    Expect to find dexpreopt/$ARCH/boot-art*.{art,oat,vdex} files.
Test: m art/build/apex/runtests.sh

Change-Id: I353ef90304bc5e18c3055ea379b3b223e5c38948

java/dexpreopt_bootjars.go

@@ -165,6 +165,8 @@ func (d *dexpreoptBootJars) GenerateBuildActions(ctx android.SingletonContext) {
 
 	// Always create the default boot image first, to get a unique profile rule for all images.
 	d.defaultBootImage = buildBootImage(ctx, defaultBootImageConfig(ctx))
+	// Create boot image for the ART apex (build artifacts are accessed via the global boot image config).
+	buildBootImage(ctx, artBootImageConfig(ctx))
 	if global.GenerateApexImage {
 		d.otherImages = append(d.otherImages, buildBootImage(ctx, apexBootImageConfig(ctx)))
 	}