Add a flag to allow extending the check post dim

Allow to keep checking attention after the screen dim event, for X
millis, where X is a DeviceConfig flag.

Also move the duration of the check pre-dim to a flag controlled
variable

Bug: 143902763
Test: manually verified + unit test updated; added tests to make sure
flag cannot cause problems

Change-Id: I94118d657c8215e07d758170749e9992c19d6535