Use WifiPickerTracker instead of WifiTracker2

WifiTracker2 is being split into separate trackers for each fragment
that needs tracking functionality (Wifi picker, Saved Networks Page,
Network Details Page). For WifiSettings2, we will use WifiPickerTracker
which differs from WifiTracker2 by only having the functionality
required for the wifi picker.

Bug: 70983952
Test: WifiSettings2Test.java
Change-Id: Ia94fa534ead7c521156d1cccbb120a345d95e020