Better align Emacs window management with Android task lifecycles

Better align Emacs window management with Android task lifecycles

* java/org/gnu/emacs/EmacsActivity.java (onCreate): Permit
overriding by child classes.
(onDestroy): Minor stylistic adjustments.
(getAttachmentToken): New function.

* java/org/gnu/emacs/EmacsMultitaskActivity.java (onCreate)
(getAttachmentToken): New functions.

* java/org/gnu/emacs/EmacsWindow.java (EmacsWindow):
<attachmentToken, preserve, previouslyAttached>: New variables.
(onActivityDetached): Remove redundant isFinishing argument.
(reparentTo): Reset the foregoing fields before registering with
the window manager.

* java/org/gnu/emacs/EmacsWindowManager.java
(EmacsWindowManager): Rename from EmacsWindowAttachmentManager.
(WindowConsumer): New function getAttachmentToken.
(isWindowEligible): New function.
(registerWindowConsumer, registerWindow, removeWindowConsumer)
(detachWindow): Implement a new window management strategy on
API 29 and subsequent releases where both varieties of toplevel
window are permanently, except when reparented, bound to the
activities to which they attach, and Emacs establishes at
strategic junctures whether those activities remain present.
(getTaskToken, pruneWindows): New functions.