Cache CoroutineScope for settings APIs

Instead of creating a new CoroutineScope each time a settings API is
called, create one scope and reuse it whenever possible.

Test: m SystemUIGoogle, atest SettingsProxyTest UserSettingsProxyTest
Bug: 330299944
Flag: NONE new async APIs and existing APIs kept unmodified.
Change-Id: I4b660cdcb49e1c618c94b1d6b2cbbf84c4b60dd4