Add WifiTrackerLib as separate dependency from SettingsLib

WifiTrackerLib should be removed from SettingsLib to avoid tying it to
SettingsLib's min_sdk_version. Modules that need WifiTrackerLib should
now explicitly add it as a static_lib.

Bug: 187099809
Test: build, make RunSettingsRoboTests
ROBOTEST_FILTER=WifiEntryPreferenceTest

Change-Id: I09b7cad0b3b17ba1a24c1200b7ea3255ca4e227f