Facilitate typing `C-SPC' on Android

* doc/emacs/android.texi (Android Windowing): Mention C-SPC
interception and how it may be disabled.

* java/org/gnu/emacs/EmacsNative.java (shouldForwardCtrlSpace):
New function.

* java/org/gnu/emacs/EmacsView.java (onKeyPreIme): New function.
If the provided key code is SPC and the event's modifier key
mask contains ControlMask, relay it directly to onKeyDown.

* java/org/gnu/emacs/EmacsWindow.java (eventModifiers): Export
and make static.

* src/android.c (shouldForwardCtrlSpace): New function.

* src/androidfns.c (syms_of_androidfns)
<android_intercept_control_space>: New defvar.

java/org/gnu/emacs/EmacsWindow.java

@@ -576,7 +576,7 @@ public final class EmacsWindow extends EmacsHandleObject
      input EVENT.  Replace bits corresponding to Left or Right keys
      with their corresponding general modifier bits.  */
 
-  private int
+  public static int
   eventModifiers (KeyEvent event)
   {
     int state;