AM: refactor LockTask whitelist update logic.

Previously, ActivityManagerService owned the LockTask package whitelist,
whereas LockTaskController contained the update logic. In this refactor,
we simplify the structure by moving the whitelist into LockTaskController.
We also add instrumentation tests to verify this behavior.

Test: bit FrameworksServicesTests:com.android.server.am.LockTaskControllerTest
Test: runtest frameworks-services
Test: go/wm-smoke
Test: cts-tradefed run cts-dev -m DevicePolicyManager --test com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_deviceOwnerUser
Bug: 63909481
Change-Id: I15ef4a446de800336f2bf9cbbd4acf6685e29320