LineageNotificationLights: Don't create KeyguardManager in constructor

* Apparently creating it so early can cause serious breakages
  that make StorageManagerService and UsbDeviceManager end up
  getting null pointer when trying to get KeyguardManager with
  ctx.getSystemService(KeyguardManager.class).
* Moving ctx.getSystemService(KeyguardManager.class) out of
  constructor to isKeyguardLocked() solves these issues.
* Also move to getSystemService(Class<T>) while at it.

Change-Id: Ib3f65ba2e726149089ab1fb12030321e0ecdeab1