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
diff --git a/packages/SettingsLib/Android.bp b/packages/SettingsLib/Android.bp
index e6492aa..23cb36f 100644
--- a/packages/SettingsLib/Android.bp
+++ b/packages/SettingsLib/Android.bp
@@ -54,6 +54,7 @@
         "SettingsLibEmergencyNumber",
         "SettingsLibTopIntroPreference",
         "SettingsLibBannerMessagePreference",
+        "SettingsLibFooterPreference",
     ],
 }