Check intent action in OMS.PackageReceiver for secure coding

NullPointerException occurs when OMS receives an intent with null action.
In constructor of OverlayManagerService, OMS.PackageReceiver is registered
with data scheme "package".
If a malicious app send broadcast intent only with data scheme "package",
NPE occurs because OMS.PackageReceiver does not check
whether intent.getAction() is null or not.
So add a logic to ignore intent with null action for secure coding.

Test: send broadcast without action like below.
 Intent intent = new Intent();
 Uri uri = Uri.parse("package:com.test");
 intent.setData(uri);
 intent.addFlags(0x01000000);
 sendBroadcast(intent);

Change-Id: I654f54a8a685de2ab985b87f53ad07c4e27db09d
Signed-off-by: Youngha Park <yh007.park@samsung.com>