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/license_sdk_test.go b/sdk/license_sdk_test.go
index 754f019..eb8112b 100644
--- a/sdk/license_sdk_test.go
+++ b/sdk/license_sdk_test.go
@@ -21,6 +21,7 @@
 )
 
 func TestSnapshotWithPackageDefaultLicense(t *testing.T) {
+	t.Parallel()
 	result := android.GroupFixturePreparers(
 		prepareForSdkTestWithJava,
 		android.PrepareForTestWithLicenses,