Fix launchMode of TvUnblockSensorActivity.

Unblock bottom sheet was showing multiple times in some apps. LaunchMode
change prevents it from doing so and now matches the mobile equivalent
of the dialog.

Test: enable camera privacy and open Duo with a camera attached to the
device.
Bug: 198129996

Change-Id: If014e8c3b6dbb5420c520b2c0ce3941d6074079e