VSR: Only ignore Composer when rate isn't changing

This refines the logic added in
I79c66faeaed262acd9c5925fe2202a9fb3f10b7b, which started ignoring
Composer-provided refresh rates on vsync callbacks when the kernel idle
timer is enabled.

This behavior is still correct when SF doesn't think that that refresh
rate has changed, but when we initiate a change, such as when the only
active layer is infrequently updating, then it's okay to rely on the
Composer-provided rate.

Bug: 158141901
Test: atest libsurfaceflinger_unittest, systrace
Change-Id: I16c87005f5cd1ca1810014211d73b4662e7c8c86