Make Extra Dim main toggle and shortcut searchable

Since the Extra Dim main toggle and shortcut preferences are created in
codes, we override `getRawDataToIndex` to return the corresponding index raw data.

Bug: 354778552
Test: atest ToggleReduceBrightColorsPreferenceTest
Test: Manually check the preferences are searchable in Settings search
Flag: com.android.settings.accessibility.fix_a11y_settings_search
Change-Id: I1642e96273cd16f35b23350af0ede3b1247ef067