Refactor for better readability of using different Bluetooth scanning

We rely on using `filter` is null or not to determine which type of
Bluetooth scanning method is going to be used (Classic of LE). Add a
variable to clearly indicate current scan type for better readibility.

Bug:289189853
Test: make RunSettingsRoboTests ROBOTEST_FILTER=DeviceListPreferenceFragmentTest
Change-Id: I85a26d61410367bd617a0194ba3bfe7ed1b03356