Added fallback strategy

FallbackBrightnessStrategy is the last resort to which the system will
fall back if none of the other strategies qualify to set the
brightness state. This strategy is also being used for evaluating the
display brightness state when we have a manually set brightness. This
is a temporary state, and the logic for evaluating the manual brightness
will soon be moved to a separate strategy

Bug: 339647371
Test: atest com.android.server.display.brightness
Change-Id: I84de9f479bf6751af7ea0954fa2b76063d77a9f9