Fix NPE in ExtServices

getSystemService(PackageManager.class) returns null.
We should call context.getPackageManager() instead.

Test: Posts a notification, no more NPE. And verified that
      smart actions feature is working.

Fixes: 139939548
Bug: 138589409

Change-Id: I8db737183ae5a7efd10a6a292c6517825da21e89