DPM: introduce setLockTaskFeatures().

Add new DPM APIs to control which SystemUI features are enabled during
LockTask mode:
* setLockTaskFeatures()
* getLockTaskFeatures()
* int flags representing various configurable SystemUI features

Bug: 65813398
Test: bit FrameworksServicesTests:com.android.server.devicepolicy.DevicePolicyManagerTest
Test: bit FrameworksServicesTests:com.android.server.am.LockTaskControllerTest
Test: cts-tradefed run cts-dev --module DevicePolicyManager -t com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_deviceOwnerUser
Change-Id: I0ee3cf6dbe2234ec29d1384195dadc0f290aa73b