Show warning toast for secure window

If a virtual display isn't secure, the screen can't be captured. Show a
warning toast if the secure window is shown on a non-secure virtual
display.

Bug: 231406802
Test: manually test with Exo. Open a secure window, observe the screen
      is black and the warning toast shows up.
Change-Id: Id518dba67e7bf4d918e14587b245eb2188444772