[Screenshot Test] Optimize Mockito usage for screenshot tests

We dont need to allocate full memory for mocked class unless we need to verify.
This CL uses `stubOnly` settings for such mocks.

Bug: 361922756
Test: Presubmit
Flag: TEST_ONLY
Change-Id: I4add1e18759dfe164c6dea537c968e0b35a66fa0
1 file changed