* doc/emacs/android.texi (Android Windowing): Revise to match.
* java/org/gnu/emacs/EmacsWindowManager.java (registerWindow)
(removeWindowConsumer, pruneWindows): Decrease minimum API for
monitoring of tasks to Android 5.0.
(getTaskToken): Ignore misleading documentation and access
baseIntent by way of RecentTaskInfo.
* java/org/gnu/emacs/EmacsActivity.java (destroy): Detach from
current window before calling finish.
* java/org/gnu/emacs/EmacsWindow.java (reparentTo): Don't clear
attachment state here...
* java/org/gnu/emacs/EmacsWindowManager.java (detachWindow):
...but do so here instead.
* 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.
