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 eea3af3f91)

wear_ui.cpp

@@ -51,8 +51,9 @@ static double now() {
 }
 
 WearRecoveryUI::WearRecoveryUI()
-    : kProgressBarBaseline(RECOVERY_UI_PROGRESS_BAR_BASELINE), menu_unusable_rows(9) {
-  // TODO: menu_unusable_rows should be computed based on the lines in draw_screen_locked().
+    : kProgressBarBaseline(RECOVERY_UI_PROGRESS_BAR_BASELINE),
+      kMenuUnusableRows(RECOVERY_UI_MENU_UNUSABLE_ROWS) {
+  // TODO: kMenuUnusableRows should be computed based on the lines in draw_screen_locked().
 
   // TODO: The following three variables are likely not needed. The first two are detected
   // automatically in ScreenRecoveryUI::LoadAnimation(), based on the actual files seen on device.
@@ -268,7 +269,7 @@ void WearRecoveryUI::StartMenu(const char* const* headers, const char* const* it
     show_menu = true;
     menu_sel = initial_selection;
     menu_start = 0;
-    menu_end = visible_text_rows - 1 - menu_unusable_rows;
+    menu_end = visible_text_rows - 1 - kMenuUnusableRows;
     if (menu_items <= menu_end) menu_end = menu_items;
     update_screen_locked();
   }