Use MockFrameTracer in unit tests

This change makes sure that MockFrameTracer is used in unittests
wherever FrameTracer logic is embedded in (mainly in
TestableSurfaceFlinger). This is to avoid double initialization of
tracing, that causes undesirable effects that are hard to debug.

Bug: 173636249
Test: libsurfaceflinger_unittest (without filter. Test the whole suite)
Change-Id: Iffa0696acde091ed4ebd300ac950544925824bea