Support desktop notifications on Android

Support desktop notifications on Android

* doc/emacs/android.texi (Android Environment): Correct list of
permissions granted by default.

* doc/lispref/os.texi (Desktop Notifications): Document the new
function `android-notifications-notify' and its limitations.

* java/AndroidManifest.xml.in: Request notification permissions.

* java/org/gnu/emacs/EmacsDesktopNotification.java: New file.

* java/res/layout/sdk8_notifications_view.xml: New file holding
substitute notification widget definitions for Android versions
prior to 3.0.

* java/res/values/strings.xml: Remove inadvertently introduced
tag attribute.

* lisp/org/org-clock.el (haiku-notifications-notify): Correct
file name in function declaration.
(android-notifications-notify): New declaration.
(org-show-notification): Use `android-notifications-notify'.

* src/androidselect.c (android_init_emacs_desktop_notification)
(android_notifications_notify_1, Fandroid_notifications_notify):
New functions.
(init_androidselect, syms_of_androidselect): Initialize new
class and define new subr.