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/cc/cc_test.go b/cc/cc_test.go
index b1c0945..7372fff 100644
--- a/cc/cc_test.go
+++ b/cc/cc_test.go
@@ -3194,12 +3194,7 @@
 
 	ctx = android.GroupFixturePreparers(
 		prepareForCcTest,
-		android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
-			if variables.BuildFlags == nil {
-				variables.BuildFlags = make(map[string]string)
-			}
-			variables.BuildFlags["RELEASE_BOARD_API_LEVEL_FROZEN"] = "true"
-		}),
+		android.PrepareForTestWithBuildFlag("RELEASE_BOARD_API_LEVEL_FROZEN", "true"),
 	).RunTestWithBp(t, bp)
 	testSdkVersionFlag("libfoo", "30")
 	testSdkVersionFlag("libbar", "29")
diff --git a/cc/prebuilt_test.go b/cc/prebuilt_test.go
index 86e6af9..acbbabc 100644
--- a/cc/prebuilt_test.go
+++ b/cc/prebuilt_test.go
@@ -476,11 +476,7 @@
 			android.FixtureRegisterWithContext(func(ctx android.RegistrationContext) {
 				android.RegisterApexContributionsBuildComponents(ctx)
 			}),
-			android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
-				variables.BuildFlags = map[string]string{
-					"RELEASE_APEX_CONTRIBUTIONS_ADSERVICES": "myapex_contributions",
-				}
-			}),
+			android.PrepareForTestWithBuildFlag("RELEASE_APEX_CONTRIBUTIONS_ADSERVICES", "myapex_contributions"),
 		)
 		ctx := testPrebuilt(t, fmt.Sprintf(bp, tc.selectedDependencyName), map[string][]byte{
 			"libbar.so": nil,
@@ -574,11 +570,7 @@
 			android.FixtureRegisterWithContext(func(ctx android.RegistrationContext) {
 				android.RegisterApexContributionsBuildComponents(ctx)
 			}),
-			android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
-				variables.BuildFlags = map[string]string{
-					"RELEASE_APEX_CONTRIBUTIONS_ADSERVICES": "myapex_contributions",
-				}
-			}),
+			android.PrepareForTestWithBuildFlag("RELEASE_APEX_CONTRIBUTIONS_ADSERVICES", "myapex_contributions"),
 		)
 		if tc.expectedErr != "" {
 			preparer = preparer.ExtendWithErrorHandler(android.FixtureExpectsAtLeastOneErrorMatchingPattern(tc.expectedErr))
@@ -638,11 +630,7 @@
 		android.FixtureRegisterWithContext(func(ctx android.RegistrationContext) {
 			android.RegisterApexContributionsBuildComponents(ctx)
 		}),
-		android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
-			variables.BuildFlags = map[string]string{
-				"RELEASE_APEX_CONTRIBUTIONS_ADSERVICES": "myapex_contributions",
-			}
-		}),
+		android.PrepareForTestWithBuildFlag("RELEASE_APEX_CONTRIBUTIONS_ADSERVICES", "myapex_contributions"),
 	)
 	ctx := testPrebuilt(t, bp, map[string][]byte{
 		"libbar.so": nil,