Prevent Android OS task trimming from deleting Emacs frames

* doc/emacs/android.texi (Android Windowing): Document proviso
on Android 7.0 and later.

* java/org/gnu/emacs/EmacsActivity.java (EmacsActivity)
<timeOfLastInteraction>: New field.
(onStop, onResume): Set and clear timeOfLastInteraction.
(isReallyFinishing): New function.
(onDestroy): Don't delete frame even in the event isFinishing
returns true if more than 4 hours have elapsed since the
activity last moved into the background.

java/org/gnu/emacs/EmacsActivity.java

@@ -20,9 +20,12 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 package org.gnu.emacs;
 
 import java.lang.IllegalStateException;
+
 import java.util.List;
 import java.util.ArrayList;
 
+import java.util.concurrent.TimeUnit;
+
 import android.app.Activity;
 
 import android.content.ContentResolver;
@@ -31,6 +34,7 @@ import android.content.Intent;
 
 import android.os.Build;
 import android.os.Bundle;
+import android.os.SystemClock;
 
 import android.util.Log;
 
@@ -78,6 +82,9 @@ public class EmacsActivity extends Activity
   /* The last context menu to be closed.  */
   private static Menu lastClosedMenu;
 
+  /* The time of the most recent call to onStop.  */
+  private static long timeOfLastInteraction;
+
   static
   {
     focusedActivities = new ArrayList<EmacsActivity> ();
@@ -271,6 +278,50 @@ public class EmacsActivity extends Activity
     syncFullscreenWith (window);
   }
 
+  @Override
+  public final void
+  onStop ()
+  {
+    timeOfLastInteraction = SystemClock.elapsedRealtime ();
+
+    super.onStop ();
+  }
+
+  /* Return whether the task is being finished in response to explicit
+     user action.  That is to say, Activity.isFinished, but as
+     documented.  */
+
+  public final boolean
+  isReallyFinishing ()
+  {
+    long atime, dtime;
+    int hours;
+
+    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.NOUGAT)
+      return isFinishing ();
+
+    /* When the number of tasks retained in the recents list exceeds a
+       threshold, Android 7 and later so destroy activities in trimming
+       them from recents on the expiry of a timeout that isFinishing
+       returns true, in direct contradiction to the documentation.  This
+       timeout is generally 6 hours, but admits of customization by
+       individual system distributors, so to err on the side of the
+       caution, the timeout Emacs applies is a more conservative figure
+       of 4 hours.  */
+
+    if (timeOfLastInteraction == 0)
+      return isFinishing ();
+
+    atime = timeOfLastInteraction;
+
+    /* Compare atime with the current system time.  */
+    dtime = SystemClock.elapsedRealtime () - atime;
+    if (dtime + 1000000 < TimeUnit.HOURS.toMillis (4))
+      return isFinishing ();
+
+    return false;
+  }
+
   @Override
   public final void
   onDestroy ()
@@ -283,7 +334,8 @@ public class EmacsActivity extends Activity
     /* The activity will die shortly hereafter.  If there is a window
        attached, close it now.  */
     isMultitask = this instanceof EmacsMultitaskActivity;
-    manager.removeWindowConsumer (this, isMultitask || isFinishing ());
+    manager.removeWindowConsumer (this, (isMultitask
+					 || isReallyFinishing ()));
     focusedActivities.remove (this);
     invalidateFocus (2);
 
@@ -340,6 +392,7 @@ public class EmacsActivity extends Activity
   onResume ()
   {
     isPaused = false;
+    timeOfLastInteraction = 0;
 
     EmacsWindowAttachmentManager.MANAGER.noticeDeiconified (this);
     super.onResume ();