Enable Recents activity in LockTask mode.

In P, we introduce DevicePolicyManager.setLockTaskFeatures() API, which
lets the DPC enable the SystemUI Recents activity in LockTask mode. To
do this, we

1) Allow recents activity in LockTaskController (otherwise it'll be
blocked from starting as it's not whitelisted)

2) Clear the RecentTasks list, so that only whitelisted tasks remain in
the list (and therefore the recents UI)

Note that 2) is done even when Recents isn't enabled through the
setLockTaskFeatures() API.

Bug: 68238765
Test: runtest systemui
Test: bit FrameworksServicesTests:com.android.server.am.LockTaskControllerTest
Test: CTS verifier > CTS verifier > Managed provisioning > Device owner tests
      > LockTask UI
Change-Id: I63c524c4d9a68b87a63cecf1642ecebd94aa2bb0