Enable configuring Emacs for "pseudo-grayscale" systems on Android

* doc/emacs/android.texi (Android Windowing): Document how to
configure Emacs for monochrome displays.

* src/androidfns.c (Fx_display_visual_class): Return
Qstatic_gray when n_planes is smaller than 24.
(Fandroid_get_connection): Set n_planes by the value of
android_display_planes.
(syms_of_androidfns): <Qstatic_gray>: New function.

* src/androidterm.c (android_alloc_nearest_color): Allocate
monochrome colors similarly to the X server.
(android_query_colors): Fix typos.
(android_draw_fringe_bitmap): Create bitmaps of n_image_planes
depth.
(android_term_init): Initialize n_image_planes to 24.
(syms_of_androidterm) <android_display_planes>: New variable.

* src/androidterm.h (struct android_display_info): New field
`n_image_planes'.

* src/image.c (n_planes) [HAVE_ANDROID]: Define to
n_image_planes.

1 files changed, 17 insertions, 2 deletions

diff --git a/src/androidfns.c b/src/androidfns.c
index 9f7ac8b69b2..b6df7ae0677 100644
--- a/src/androidfns.c
+++ b/src/androidfns.c
@@ -1345,7 +1345,12 @@ DEFUN ("x-display-visual-class", Fx_display_visual_class,
        doc: /* SKIP: real doc in xfns.c.  */)
   (Lisp_Object terminal)
 {
-  check_android_display_info (terminal);
+  struct android_display_info *dpyinfo;
+
+  dpyinfo = check_android_display_info (terminal);
+
+  if (dpyinfo->n_planes < 24)
+    return Qstatic_gray;
 
   return Qtrue_color;
 }
@@ -1805,7 +1810,16 @@ Android, so there is no equivalent of `x-open-connection'.  */)
   terminal = Qnil;
 
   if (x_display_list)
-    XSETTERMINAL (terminal, x_display_list->terminal);
+    {
+      XSETTERMINAL (terminal, x_display_list->terminal);
+
+      /* Update the display's bit depth from
+         `android_display_planes'.  */
+      x_display_list->n_planes
+        = (android_display_planes > 8
+           ? 24 : (android_display_planes > 1
+                   ? android_display_planes : 1));
+    }
 
   return terminal;
 #endif
@@ -3479,6 +3493,7 @@ syms_of_androidfns (void)
 {
   /* Miscellaneous symbols used by some functions here.  */
   DEFSYM (Qtrue_color, "true-color");
+  DEFSYM (Qstatic_gray, "static-color");
   DEFSYM (Qwhen_mapped, "when-mapped");
 
   DEFVAR_LISP ("x-pointer-shape", Vx_pointer_shape,