Add PrepareForTestWithBuildFlag

Add a helper function that creates a test fixture preparer that
sets a build flag, and use it everywhere that was setting build flags
manually.

Test: all soong tests
Flag: EXEMPT refactor
Change-Id: I68d50d68787a30d091f0827e8caa51f5c5a762ef
diff --git a/java/hiddenapi_singleton_test.go b/java/hiddenapi_singleton_test.go
index 6229797..afe8b4c 100644
--- a/java/hiddenapi_singleton_test.go
+++ b/java/hiddenapi_singleton_test.go
@@ -203,10 +203,8 @@
 				FixtureConfigureBootJars("platform:foo"),
 				android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
 					variables.Always_use_prebuilt_sdks = proptools.BoolPtr(tc.unbundledBuild)
-					variables.BuildFlags = map[string]string{
-						"RELEASE_HIDDEN_API_EXPORTABLE_STUBS": "true",
-					}
 				}),
+				android.PrepareForTestWithBuildFlag("RELEASE_HIDDEN_API_EXPORTABLE_STUBS", "true"),
 			).RunTestWithBp(t, `
 		java_library {
 			name: "foo",