SF: Deduplicate mock scheduler setup in tests

Extract TestableSurfaceFlinger::setupMockScheduler, and add an option to
customize the PhysicalDisplayId of the injected RefreshRateSelector (and
its display modes), so that tests can later inject a DisplayDevice with
a matching ID. Remove the DisplayModesVariant with two display modes, as
the few tests that used it are unrelated to scheduling and will later be
decoupled entirely.

Bug: 259436835
Bug: 241285191
Test: libsurfaceflinger_unittest
Change-Id: Ie58b6947201a1911342e7bcd5ea64dd8702aaa57
Merged-In: Ie58b6947201a1911342e7bcd5ea64dd8702aaa57
(cherry picked from commit fb281c53)