Allow userActivityTimeout override when keyguard is showing.

Recently changed to not allowing timeout overriding if the keyguard
is showing.  This prevented keyguard itself from setting a quicker
timeout for user activity.

This CL goes back to testing to see if a higher-z-order window is
the keyguard instead of blanket "isKeyguardShowing" test.

Bug: 149673668
Test: Manual - timeout correct when device unlocked, keyguard, or
showing notification shade (locked & unlocked).

Change-Id: Idd18463fe1f42daeba8116cff48ccc65e95fe9a1