AM: introduce ActivityOptions.setLockTaskMode().

This new API allows any activity to be launched into LockTask mode, as
if the package of the activity has declared "lockTaskMode =
if_whitelisted" in its manifest.

The lockTaskLaunchMode logic is also moved from TaskRecord to
ActivityRecord, in order to accommodate the new path from
ActivityOptions.

Bug: 66124180
Test: cts-tradefed run cts-dev --module DevicePolicyManager -t com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_deviceOwnerUser
Test: bit FrameworksServicesTests:com.android.server.am.LockTaskControllerTest
Test: manual, with TestDPC's kiosk mode
Change-Id: I786bf194ed20a212bbad1f3cdb9394cc77aa4d77