Remove location_modes_previous references

1. Remove reference to LOCATION_MODE_PREVIOUS.

2. Add setLocationEnabled method in LocationEnabler. When turning location
ON/OFF from Settings app or Quick Settings, use
LocationEnabler.setLocationEnabled() instead of setLocationMode().

3. Change unit tests accordingly.

Bug: 70990911
Test: Robolectric
Test: Manual
Change-Id: Ic02ef3cd02f9aa7d2ef18697b19b507575aaf5c2