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

java/bootclasspath_fragment_test.go

@@ -222,11 +222,7 @@ func TestBootclasspathFragment_StubLibs(t *testing.T) {
 		PrepareForTestWithJavaSdkLibraryFiles,
 		FixtureWithLastReleaseApis("mysdklibrary", "myothersdklibrary", "mycoreplatform"),
 		FixtureConfigureApexBootJars("someapex:mysdklibrary"),
-		android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
-			variables.BuildFlags = map[string]string{
-				"RELEASE_HIDDEN_API_EXPORTABLE_STUBS": "true",
-			}
-		}),
+		android.PrepareForTestWithBuildFlag("RELEASE_HIDDEN_API_EXPORTABLE_STUBS", "true"),
 	).RunTestWithBp(t, `
 		bootclasspath_fragment {
 			name: "myfragment",