Facilitate locating the app library directory

Facilitate locating the app library directory

* doc/emacs/android.texi (Android File System): Document where
the app library directory can probably be found.
* src/android.c (android_create_lib_link): New function.  Try to
symlink `lib' in the directory holding the files directory to
the app library directory.
(setEmacsParams): Call that function if Emacs is being
initialized from an application context.