Fix mDeskModeKeepsScreenOn logic

If mDeskModeKeepsScreenOn was set to 1, the screen still timeouted because
mCurUiMode != Configuration.UI_MODE_TYPE_DESK. Given that mCurUiMode is
a bitmask field also consisting of UI_MODE_NIGHT_MASK, fix this check
so the behaviour is corrected.

See core/java/android/content/res/Configuration.java for the explanation
on uiMode's values

Change-Id: Ic7a0fd375dd4700eacb48974e20ad387732fc1d5