Set secure window flag for AuthContainerView

Marks the window in which AuthContainerView is shown as secure in order
to prevent it from being screenshotted. This matches the behavior of
ConfirmDeviceCredentialActivity in Settings.

Test: atest AuthContainerViewTest
Test: Manual:
1. Use TestDPC to set up work profile
2. Set a PIN for the work profile in Settings > Security
3. Lock and unlock the device with non-work PIN
4. Launch a work app to be prompted for the work PIN
5. Attempt to take a screenshot

Before: Able to take a screenshot of the credential activity

After: Not able to take a screenshot

Fixes: 148000618
Change-Id: Ib1ac94915d28d8d88ec6a81292b69db396c419f9