Stop displaying "can't dream" debug message on every power button press.

The debug message is now behind a DEBUG_DREAMS compile-time constant
that defaults to false.

Bug: 430053930
Test: Manually by verifying debug message only displays if DEBUG_DREAMS
is true.
Flag: EXEMPT bugfix

Change-Id: Ic38137d53b42a748395270114f4091a377fdd7d8