Noted that OnSharedPreferenceChangeListener does not store a strong reference

to the listener.

Because of this, the listener is susceptible to garbage collection unless the
caller preserves a strong reference to it. We suggest that the caller store a
reference to the listener in an instance field of an object that will exist as
long as the listener is needed.

Also fixed code sample formatting in Settings.jd (some lines were overlong).

Docs are staged in:

http://asolovay.mtv:8901/guide/topics/ui/settings.html#Listening

http://asolovay.mtv:8901/reference/android/content/SharedPreferences.html#registerOnSharedPreferenceChangeListener(android.content.SharedPreferences.OnSharedPreferenceChangeListener)

Bug: 10437648
Change-Id: I48f5b87fa2ca7a240661d0cc00114d225819d54d