Remove WifiTrackerLib from SettingsLib

SettingsLib should not depend on WifiTrackerLib since WifiTrackerLib's
min_sdk_level will keep updating to use the latest Wifi APIs. Since many
other apps use SettingsLib and may not have their min_sdk_level updated,
we should make sure WifiTrackerLib's min_sdk_level is decoupled from
SettingsLib's.

Bug: 187099809
Test: build
Change-Id: I5e530e9b714870e61e519c2eaccc6710683a66d3