Add bootclasspath_fragment as an alias for boot_image

This is part of the work to rename boot_image to bootclasspath_fragment
which is being done for two reasons:
1. To avoid clashing with the bootimg module type.
2. To better reflect what this represents.

While a bootclasspath_fragment can create what ART calls a boot image
(which is different to what the bootimg module type represents) it does
not have to do so.

Bug: 177892522
Test: m nothing
Change-Id: Ib45604be7adc790ded9e27a2ac812dd7522ca8db
diff --git a/java/boot_image_test.go b/java/boot_image_test.go
index f3d603d..37906ff 100644
--- a/java/boot_image_test.go
+++ b/java/boot_image_test.go
@@ -41,6 +41,18 @@
 		`)
 }
 
+func TestUnknownBootclasspathFragmentImageName(t *testing.T) {
+	prepareForTestWithBootImage.
+		ExtendWithErrorHandler(android.FixtureExpectsAtLeastOneErrorMatchingPattern(
+			`\Qimage_name: Unknown image name "unknown", expected one of art, boot\E`)).
+		RunTestWithBp(t, `
+			bootclasspath_fragment {
+				name: "unknown-boot-image",
+				image_name: "unknown",
+			}
+		`)
+}
+
 func TestUnknownPrebuiltBootImage(t *testing.T) {
 	prepareForTestWithBootImage.
 		ExtendWithErrorHandler(android.FixtureExpectsAtLeastOneErrorMatchingPattern(