Moves lock screen shortcuts summary loading to the background.

To load the summary for the Settings > Display > Lock screen >
Shortcuts, we need to issue a binder/IPC call to query the content
provider that resides in the System UI process.

Previously, this was being done on the main thread. The thinking was
that it's critical to load this as soon as the Settings page is shown.
This results in ANRs, as per the attached bug.

The fix is to move the loading off the main thread and onto a background
thread.

Fix: 274788437
Test: unit tests updated
Test: manually verified that the summary is correct and that it updates
correctly after clicking on it, changing the lock screen shortcuts, and
going back

Change-Id: I41a5e883236de4f16c105f3930b8849538807f00