Explicitly define default actions for mocks in VirtualCameraSessionTest.

GMock will by default use the ScopedAStatus constructor, which will
construct ScopedAStatus wrapping nullptr status and crash when
someone attempts to print it using << operator (which gunit does).

Bug: 25482184
Test: atest virtual_camera_tests
Change-Id: I34013792941f90fcab9680bafec9c3b5f95c7880