Const modifiers

This functions do not change class variables
Would be good to mark them as const, so
class variables are not changed by coincidence

Change-Id: Iea34f6d26dbd1bde813035160e07ff2a681989e6
diff --git a/screen_ui.cpp b/screen_ui.cpp
index 706877b..bb2772d 100644
--- a/screen_ui.cpp
+++ b/screen_ui.cpp
@@ -98,7 +98,7 @@
     }
 }
 
-int ScreenRecoveryUI::PixelsFromDp(int dp) {
+int ScreenRecoveryUI::PixelsFromDp(int dp) const {
     return dp * density_;
 }
 
@@ -256,12 +256,12 @@
     *y += 4;
 }
 
-void ScreenRecoveryUI::DrawTextLine(int x, int* y, const char* line, bool bold) {
+void ScreenRecoveryUI::DrawTextLine(int x, int* y, const char* line, bool bold) const {
     gr_text(gr_sys_font(), x, *y, line, bold);
     *y += char_height_ + 4;
 }
 
-void ScreenRecoveryUI::DrawTextLines(int x, int* y, const char* const* lines) {
+void ScreenRecoveryUI::DrawTextLines(int x, int* y, const char* const* lines) const {
     for (size_t i = 0; lines != nullptr && lines[i] != nullptr; ++i) {
         DrawTextLine(x, y, lines[i], false);
     }