[ITS][Night] add default brightness

Details: earlier testing required finding a dot at any brightness to
continue with testing. Updated find_tablet_brightness to use the default
testing brightness if the orientation dot cannot be found.
Bug: 294569847
Test: tested locally 3x with Pixel, another group on another Pixel reproduced PASSing result
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:29f844fc22f3e34844605d932ad581a5b3680ffd)
Merged-In: I834ead951e79c63eca0b47ad0fda02e91987dc68
Change-Id: I834ead951e79c63eca0b47ad0fda02e91987dc68