Remove activity from task if not set properly.

We do not start an activity in the case of a lock task mode
violation. However, the activity is still associated with the task
and therefore participates in future interactions, such as
visibility checks.

This changelist addresses this issue by removing the assocation in
the case of failures.

Change-Id: Ibf7edd2bd4532de5e7f355311a808ffd21e542e3
Fixes: 35034729
Fixes: 35035258
Fixes: 34179495
Test: cts-tradefed run cts-dev --module DevicePolicyManager --test com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_deviceOwnerUser