Flag WindowManager refresh rate fields for test

Even new TestApi is also required to have FlaggedApi tag.

Bug: 304475199
Test: adb shell device_config put core_graphics
android.view.flags.wm_display_refresh_rate_test true
Test: atest CtsGraphicsTestCases --test-filter="FrameRateOverrideTest*"

Change-Id: I8fdfbd06c3609b7d6ee3746b99cf98cdc21ff6d6