Fix split-screen visible apps issue when screen is rotated on lockscreen

When the keyguard is showing we say the docked stack is invisible so that
any activity that displays on top of the keyguard isn't cropped by the
docked stack. However this causes issues when the docked stack is resized
due to orientation change while the keyguard is showing.
We now ignore the visibility state of the docked stack when trying to get
bounds information for resizing while the keyguard is showing.

Bug: 25970565
Change-Id: I7479a1d0afed3b2edfb17848c56c5c3902b3709e