Add experiment flag for device account detection.

Device account detection does DB queries that could potentially block the UI
thread. Just returning the "null" account as the only device account is fine
for now.

Test
* manually verify that "device" account still shows in editor and default
  account chooser when no other accounts exist
* manually verify that "device" account still shows in nav drawer when another
  non-google account is added
* manually verify that "device" account is hidden after Google Account is
  added

Bug 32312206

Change-Id: I347e9dcf0aa6252c7b0be79fea7757b03569edf7