[Settings] Data roaming warning only when turn ON

handlePreferenceTreeClick() goes after setChecked() however
configuration may have been altered within setChecked().

Follow same design within MobileDataPreferenceController to fix this
issue. A boolean state will be maintained as the condition when warning
dialog is required when click.

Bug: 148491064
Test: m RunSettingsRoboTests -j ROBOTEST_FILTER=RoamingPreferenceControllerTest
Change-Id: I63c4e2149b0e9efa16fd9ee402652c67eb9c5a4d