Don't focus search user-specified views which are invisible

While searching for next user-specified focus, skip over any
views which are not visible. This will continue searching down
the chain until it either finds a visible view or until the
chain ends (at which point the normal findNextFocus search is
used).

This was causing problems in apps like the wallpaper selector
where focus would get stuck because calling requestFocus on an
invisible view is a no-op.

Bug: 37789849
Test: Added CTS. Checked that focus no-longer gets stuck in
      wallpaper app.

Change-Id: I1a52143bd6c63364418fe6250e6c3f83e2e1c726