Settings: Add a setting to toggle wifi scan throttling

Scan throttling affects apps which are used by testers for survey-like
application. Add a hidden setting which can be toggled via adb commands
to turn off all scan throttling.

Bug: 111763749
Test: Unit tests
Test: `adb shell settings put global wifi_scan_throttle_enabled 0` on
user build.
Change-Id: I6c9a468c573239914b88de0afcdd7e477429cb16