Fix RobolectricPackageManager.LaunchIntentForPackage() issue.

I found the bug that `RobolectricPackageManager.LaunchIntentForPackage()` always returns non-null object, while actual `PackageManager.LaunchIntentForPackage()` returns `null` when the corresponding package is not installed. I fixed by almost copying the code from Android source. Also I added a test case accordingly.