UpdatesActivity: only set performance mode on A/B devices

If the preference dialog gets dismissed on a non-ab-device it still
tries to set performance mode.

Add a check to prevent that and only set performance mode on A/B devices.

Change-Id: I6290eb80550b4cfc8a012f94bdf228af7d4fc416
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>