Save build timestamps in Android builds

Save build timestamps in Android builds

* java/Makefile.in (install_temp/assets/build_info): New
rule.:(emacs.apk-in): Depend on that file.
* lisp/version.el (android-read-build-system)
(android-read-build-time): New functions.
(emacs-build-system, emacs-build-time): Use those functions on
Android, as dumping is done after installation on Android.
* src/fileio.c (Finsert_file_contents):
* src/window.c (replace_buffer_in_windows): Don't call functions
if they are not defined, which can happen during loadup.