Add option to override defaultManifestVersion

Add an option to override defaultManifestVersion using environment
variable. The environment variable will be used to override the apex
version locally when developing a desert release feature in
mainline-prod branch. Overriding the apex version in mainline-prod
branch allows us to install apex built in mainline-prod branch to device
running code from git_master which has a higher apex version than
mainline-prod branch.

Bug: b/233608815
Bug: b/196860838
Test: presubmit
Test: go test -v ./build/soong/apex/
Change-Id: I8ac2aae2b6b29da7f09db6edb893e0a0f0691e51

apex/apex_test.go

@@ -4130,6 +4130,41 @@ func TestApexName(t *testing.T) {
 	ensureNotContains(t, androidMk, "LOCAL_MODULE := mylib.com.android.myapex\n")
 }
 
+func TestOverrideApexManifestDefaultVersion(t *testing.T) {
+	ctx := testApex(t, `
+		apex {
+			name: "myapex",
+			key: "myapex.key",
+			apex_name: "com.android.myapex",
+			native_shared_libs: ["mylib"],
+			updatable: false,
+		}
+
+		apex_key {
+			name: "myapex.key",
+			public_key: "testkey.avbpubkey",
+			private_key: "testkey.pem",
+		}
+
+		cc_library {
+			name: "mylib",
+			srcs: ["mylib.cpp"],
+			system_shared_libs: [],
+			stl: "none",
+			apex_available: [
+				"//apex_available:platform",
+				"myapex",
+			],
+		}
+	`, android.FixtureMergeEnv(map[string]string{
+		"OVERRIDE_APEX_MANIFEST_DEFAULT_VERSION": "1234",
+	}))
+
+	module := ctx.ModuleForTests("myapex", "android_common_com.android.myapex_image")
+	apexManifestRule := module.Rule("apexManifestRule")
+	ensureContains(t, apexManifestRule.Args["default_version"], "1234")
+}
+
 func TestCompileMultilibProp(t *testing.T) {
 	testCases := []struct {
 		compileMultiLibProp string