[SettingsProvider] new APIs to set and reset System settings to default

As requested, adding system APIs to 1) set system settings values as default before a
test and 2) reset the values to default after the test.

BUG: 279083734
Test: atest android.provider.cts.settings.Settings_SystemTest#testResetToDefaults
Change-Id: I675b6388cde78fcacf8b70e187dfefcb2a416fad