Fix frame when system status bar is hidden

When the system status bar is hidden, UIApplication.shared.statusBarFrame returns a zero frame.

VolumeBar sets the internal frame of its window and view when start is called. Since it previously used UIApplication.shared.statusBarFrame, which could be zero when start is called, this could cause an incorrect layout. Now we use UIScreen.main.bounds instead (and wrap all layout parameters in abs just in case).

We can't use UIApplication.shared.keyWindow?.bounds because the VolumeBar might be started before the key window is available. Note that the use of UIScreen.main.bounds won't work on iPad in split-screen, but VolumeBar doesn't currently work on iPad at all.