Lazy-load bitmap resources

[mattrossman]

Summary

• Lazy-load bitmap resources instead of creating them at startup.
• This should apply broadly, not just to the battery charging or Bluetooth icons.
• The goal is to reduce baseline heap usage while keeping rendering behavior unchanged.

Rationale

• Current logs show the status bar eagerly loads multiple bitmaps up front.
• Static palette cleanup only saved a small amount, so startup bitmap residency is still the meaningful win.
• Most bitmaps in this app are only visible in specific states, so deferring creation until first use should help overall heap pressure.

Pattern

static GBitmap *s_icon;
static GColor s_icon_palette[2];

static void ensure_icon_loaded(void) {
    if (!s_icon) {
        s_icon = gbitmap_create_with_resource(RESOURCE_ID_IMAGE_ICON);
        gbitmap_set_palette(s_icon, s_icon_palette, false);
    }
}

static void maybe_unload_icon(bool should_show) {
    if (!should_show && s_icon) {
        gbitmap_destroy(s_icon);
        s_icon = NULL;
    }
}

static void layer_update_proc(Layer *layer, GContext *ctx) {
    bool should_show = should_show_icon();
    if (!should_show) {
        maybe_unload_icon(false);
        return;
    }

    ensure_icon_loaded();
    graphics_draw_bitmap_in_rect(ctx, s_icon, ICON_FRAME);
}

static void state_changed_callback(bool active) {
    layer_mark_dirty(s_icon_layer);
    maybe_unload_icon(active);
}

This pattern fits state-driven icons like Bluetooth connected/disconnected, battery charging, and quiet time.