RemotePreferenceManager: Fix receiver registration issues.

- We might get see attach() calls from multiple prefs, so stop assuming
  there's only one per key. Namely, a new activity might be created and
  attached before the previous one is destroyed
- The passed-in context is likely to be an activity one. As the manager
  instance is static, this can cause leaks and the receiver might be
  attached to a destroyed activity. Just hold onto the application
  context instead, which is a singleton anyway.

Change-Id: Ica17c19ab6e1cdfd402968ce2594d3ed362e23ee
JIRA:LINN-3