SysUI: fix Home and Recents button in LockTask.

Previously, we couldn't disable the Recents button independently from
the Home button in LockTask mode. This was intentional so that the user
could exit screen pinning mode by the Back + Recents gesture.  However
this logic didn't distinguish between screen pinning and LockTask modes.

This CL removes this constraint for LockTask mode, and ensures both Back
and Recents buttons are shown for screen pinning mode.

Bug: 68273844
Test: CTS verifier > Managed provisioning > Device owner tests
      > LockTask UI
Test: runtest systemui

Change-Id: I252dd802c4862b76b12ba12b0205ab107a74f212