[CS] Remove CS#isKeyguardShowing from OverviewProxyRecentsImpl.

Bug: 277764509
Test: while unlocked, tap recents button on 3 button nav > verify
recents opens successfully
Test: open an occluding activity over lockscreen (e.g. QR code scanner
via shortcut), tap recents button on 3 button nav > verify fingerprint
auth appears instead of recents. Verify recents appears once auth
succeeds.

Change-Id: Ic736fc7657cc7e27ee13c44ade887429ea2bffdf