SF: add a new behaviour for idle timer on VRR

When idle timer times out on VRR, change the refresh rate
indicator to show "- -". The render rate doesn't cange as a result
of idleness.

Bug: 333443503
Test: manual
Flag: EXEMPT bugfix
Change-Id: Ie4f51a2a9da1a5e229b3504881117b12f1fd1b6a