FinishActivity: prevent jumping layout when finishing

When setting up the finishing sequence, the navigation bar gets
hidden, resulting in the layout adjusting and moving.

Once the start button is pressed, pressing back on the navigation
bar is ignored.

Change-Id: Ibb961ab059c2b8be38e88db691d4293e180f3b1c
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>