Add t.Parallel() to SDK tests

Reduces time to run go test ./sdk from 44 seconds to 22.5 seconds.

Requires moving snapshotTestCustomizations from a map to a struct
with individual fields to avoid concurrent map accesses between
subtests.

Test: go test ./sdk
Change-Id: Id6f451ba9cbace8bc7ea915033a795456b85cf3f
diff --git a/sdk/sdk_test.go b/sdk/sdk_test.go
index 2532a25..b525eb8 100644
--- a/sdk/sdk_test.go
+++ b/sdk/sdk_test.go
@@ -40,6 +40,7 @@
 // Ensure that prebuilt modules have the same effective visibility as the source
 // modules.
 func TestSnapshotVisibility(t *testing.T) {
+	t.Parallel()
 	packageBp := `
 		package {
 			default_visibility: ["//other/foo"],
@@ -160,6 +161,7 @@
 }
 
 func TestSdkInstall(t *testing.T) {
+	t.Parallel()
 	sdk := `
 		sdk {
 			name: "mysdk",
@@ -326,6 +328,7 @@
 
 // Ensure that sdk snapshot related environment variables work correctly.
 func TestSnapshot_EnvConfiguration(t *testing.T) {
+	t.Parallel()
 	bp := `
 		sdk {
 			name: "mysdk",
@@ -352,6 +355,7 @@
 	}
 
 	t.Run("no env variables", func(t *testing.T) {
+		t.Parallel()
 		result := preparer.RunTest(t)
 
 		checkZipFile(t, result, "out/soong/.intermediates/mysdk/common_os/mysdk-current.zip")
@@ -377,6 +381,7 @@
 	})
 
 	t.Run("SOONG_SDK_SNAPSHOT_TARGET_BUILD_RELEASE=S", func(t *testing.T) {
+		t.Parallel()
 		result := android.GroupFixturePreparers(
 			prepareForSdkTestWithJava,
 			java.PrepareForTestWithJavaDefaultModules,
@@ -468,6 +473,7 @@
 	})
 
 	t.Run("test replacing exportable module", func(t *testing.T) {
+		t.Parallel()
 		result := android.GroupFixturePreparers(
 			prepareForSdkTestWithJava,
 			java.PrepareForTestWithJavaDefaultModules,