Introduce boolean flag to show/hide Location Scanning preference.

This introduce one new flag:
config_show_location_scanning

Which when set to false, will hide "Scanning" in Location preference.

Some OEMs may control Bluetooth/Wi-Fi scanning themselves, so they may
want to hide the page from users.

Bug: 62418710
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=LocationScanningPreferenceControllerTest

Change-Id: I19f0020a2c94f70b3b7956529f57af098ccc79fd