SysUI: Use mScreenOnFromKeyguard for panel visibility

We currently depend on mScreenOn when determining whether the panel
is visible to the user. However, this variable is updated via a
broadcast and might hence be slightly out-of-date with other keyguard
state.

Bug: 18067294
Change-Id: I51078b4a470db9a5a1f639e84ac65f6066c5669c