Make a layer secure if its parent is secure.

Dialogs and context menus can be captured in screenshots even if their
parent layer/window is marked secure. Therefore we should let the secure
flag be inherited from their parents as well.

Bug: 169851503
Test: Screen capture can't capture dialogs in a window marked secure.
Test: atest libsurfaceflinger_unittest
Test: atest SurfaceFlinger_test:ScreenCaptureTest
Change-Id: I0f32a03aeb733682df787105b051c691d1da7bc6