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

android/testing.go

@@ -174,6 +174,16 @@ var PrepareForTestDisallowNonExistentPaths = FixtureModifyConfig(func(config Con
 	config.TestAllowNonExistentPaths = false
 })
 
+// PrepareForTestWithBuildFlag returns a FixturePreparer that sets the given flag to the given value.
+func PrepareForTestWithBuildFlag(flag, value string) FixturePreparer {
+	return FixtureModifyProductVariables(func(variables FixtureProductVariables) {
+		if variables.BuildFlags == nil {
+			variables.BuildFlags = make(map[string]string)
+		}
+		variables.BuildFlags[flag] = value
+	})
+}
+
 func NewTestArchContext(config Config) *TestContext {
 	ctx := NewTestContext(config)
 	ctx.preDeps = append(ctx.preDeps, registerArchMutator)