Fix a SecurityExcption when trying to start low-light dream.

A SecurityException can occur when attempting to start a dream with a
power button press when in low-light conditions. This can happen because
both a "regular" dream and the low-light dream are started at the same
time.

Test: manually by docking, covering the light sensor, and pushing the
power button to start dreaming. Low-light should start without any
crashes.

Bug: 269947573
Change-Id: Ie1fbb9fce9f34be6b86bc4738f64c871bed99e2e