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

java/hiddenapi_singleton_test.go

@@ -50,61 +50,6 @@ func TestHiddenAPISingleton(t *testing.T) {
 	android.AssertStringDoesContain(t, "hiddenapi command", hiddenapiRule.RuleParams.Command, want)
 }
 
-func TestHiddenAPIIndexSingleton(t *testing.T) {
-	result := android.GroupFixturePreparers(
-		hiddenApiFixtureFactory,
-		PrepareForTestWithJavaSdkLibraryFiles,
-		FixtureWithLastReleaseApis("bar"),
-		FixtureConfigureBootJars("platform:foo", "platform:bar"),
-	).RunTestWithBp(t, `
-		java_library {
-			name: "foo",
-			srcs: ["a.java"],
-			compile_dex: true,
-
-			hiddenapi_additional_annotations: [
-				"foo-hiddenapi-annotations",
-			],
-		}
-
-		java_library {
-			name: "foo-hiddenapi-annotations",
-			srcs: ["a.java"],
-			compile_dex: true,
-		}
-
-		java_import {
-			name: "foo",
-			jars: ["a.jar"],
-			compile_dex: true,
-			prefer: false,
-		}
-
-		java_sdk_library {
-			name: "bar",
-			srcs: ["a.java"],
-			compile_dex: true,
-		}
-	`)
-
-	hiddenAPIIndex := result.SingletonForTests("hiddenapi_index")
-	indexRule := hiddenAPIIndex.Rule("singleton-merged-hiddenapi-index")
-	CheckHiddenAPIRuleInputs(t, `
-.intermediates/bar/android_common/hiddenapi/index.csv
-.intermediates/foo/android_common/hiddenapi/index.csv
-`,
-		indexRule)
-
-	// Make sure that the foo-hiddenapi-annotations.jar is included in the inputs to the rules that
-	// creates the index.csv file.
-	foo := result.ModuleForTests("foo", "android_common")
-	indexParams := foo.Output("hiddenapi/index.csv")
-	CheckHiddenAPIRuleInputs(t, `
-.intermediates/foo-hiddenapi-annotations/android_common/javac/foo-hiddenapi-annotations.jar
-.intermediates/foo/android_common/javac/foo.jar
-`, indexParams)
-}
-
 func TestHiddenAPISingletonWithSourceAndPrebuiltPreferredButNoDex(t *testing.T) {
 	expectedErrorMessage :=
 		"hiddenapi has determined that the source module \"foo\" should be ignored as it has been" +