AM: remove toasts for LockTask mode.

Previously, toasts like "Screen pinned", "Screen unpinned", "This app
can't be pinned" are shown during the LockTask mode. These messages are
unncecessary for this use case, so we remove them. They will now only be
shown for the user-initiated screen pinning mode.

Bug: 67934620
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: I2dd1a95afac207040263eadafd459a90de6aa9eb