Allow framework-atb-backward-compatibility to be added to bootclasspath

Fixes a build failure that occurs when it is executed with
REMOVE_ATB_FROM_BCP=true.

Setting REMOVE_ATB_FROM_BCP=true adds the
framework-atb-backward-compatibility target to the PRODUCT_BOOT_JARS
path in place of android.test.base. That causes a problem because the
build expects that everything on the PRODUCT_BOOT_JARS is installable.

Bug: 73711752
Test: make droid and flash both with and without REMOVE_ATB_FROM_BCP=true
Change-Id: I6595ff8dba42e6b85abbf6a6cad4d8970b5a91a0

Android.bp

@@ -768,6 +768,7 @@ java_library_host {
 // specified on the build command line.
 java_library {
     name: "framework-atb-backward-compatibility",
+    installable: true,
     srcs: [
         "core/java/android/content/pm/AndroidTestBaseUpdater.java",
     ],