SF: don't touch boost if we only have Exact layers

When we only have layers with vote type Exact (those are apps
that set a preferredDisplayModeId), we shouldn't let touch
to change the display refresh rate as there are no layers that
are interested in that switch.

Test: SF unit tests
Bug: 190331974
Change-Id: I23226a806d420dc3ee5a2f7dd77d5f39838f2745