Move hidden API index file rule to platform_bootclasspath

This change moves the monolithic hidden API index file creation rule
from the hiddenAPIIndexSingleton into the platform_bootclasspath. It
also moves the corresponding test from java/hiddenapi_singleton_test.go
to java/platform_bootclasspath_test.go.

Bug: 179354495
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Ia295d0f7ae9b51ea816f16921aa42339ed91704e
diff --git a/apex/apex_test.go b/apex/apex_test.go
index f39c7e3..977a954 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -4566,8 +4566,8 @@
 
 	checkHiddenAPIIndexInputs := func(t *testing.T, ctx *android.TestContext, expectedInputs string) {
 		t.Helper()
-		hiddenAPIIndex := ctx.SingletonForTests("hiddenapi_index")
-		indexRule := hiddenAPIIndex.Rule("singleton-merged-hiddenapi-index")
+		platformBootclasspath := ctx.ModuleForTests("platform-bootclasspath", "android_common")
+		indexRule := platformBootclasspath.Rule("platform-bootclasspath-monolithic-hiddenapi-index")
 		java.CheckHiddenAPIRuleInputs(t, expectedInputs, indexRule)
 	}