Fixing DisplayTest for VRR display

OverlayDisplay created using defauld display refresh rate and config.
For VRR default display, overlay display is inheriting parent property and might have synthetic modes added.
Both testMode and testRefreshRateSwitchOnSecondaryDisplay use overlay display for mode check and expect overlay display to have 2 modes, however depending on default display, synthetic modes might be added
Filtering synthetic modes, to make tests predictable

Bug: b/369467002
Test: atest DisplayTest
Flag: EXEMPT test only
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:87e66a371d20913c5cd1c403229c4e7e33ee8557)
Merged-In: Idd26eab8eb1d47c3e7aef1947d1f5b4234a06d25
Change-Id: Idd26eab8eb1d47c3e7aef1947d1f5b4234a06d25