wear_ui: Expose menu_unusable_rows via Makefile var.

This variable is useful on small screens (e.g. on watches) to handle
long menus. We should have better way to handle this value smartly.
Prior to that, expose the value to be overridable by using the generic
wearable UI module (librecovery_ui_wear).

Bug: 64307776
Test: Define the variable, build and boot into recovery image and check
      the UI menu.
Change-Id: I5d7a6baa8bb4cc852bfcc2a7b3cc9686c1c8817e
(cherry picked from commit eea3af3f911d36ac1a82a9fb95d24912cc07e3b1)
diff --git a/wear_ui.h b/wear_ui.h
index c2bbdb6..3bd90b6 100644
--- a/wear_ui.h
+++ b/wear_ui.h
@@ -46,7 +46,7 @@
 
   // Unusable rows when displaying the recovery menu, including the lines for headers (Android
   // Recovery, build id and etc) and the bottom lines that may otherwise go out of the screen.
-  int menu_unusable_rows;
+  const int kMenuUnusableRows;
 
   int GetProgressBaseline() const override;