inputflinger: remove viewport fake for device context

Now that InputMapperUnitTest::mDevice is a mock, we can simply mock the
getAssociatedViewport call instead.

Bug: 319217713
Test: atest frameworks/native/services/inputflinger/tests/CursorInputMapper_test.cpp
Test: atest frameworks/native/services/inputflinger/tests/RotaryEncoderInputMapper_test.cpp
Flag: TEST_ONLY
Change-Id: I81246b29a6be0e8c9b60f9f0cf690754e26e5321