Add start-encryption intent filter to settings

* Add intent-filter for "android.app.action.START_ENCRYPTION"
* Add pseudo-activity to host intent-filter
* Add code to settings page that exits quickly when encryption
  is not available or is already started.

This can be tested using ApiDemos -> App -> Device Admin

Bug: 3346770
Change-Id: Ie97459cf9e2a7b09b690bf085e64ef905466e77a