Extract footer preference as a widget lib

Currently, developer needs to import SettingsLib to use
footer preference. But the size of SettingsLib is too huge.

We extract the footer preference as a lib so that developer
only needs to import a small lib for using footer.

Test: Rebuilt apk, and see the footer preference
Fix: 173088235
Change-Id: I35ba99a49c7a13542d79f0340eecdbe9844102d0