ui: Read custom recovery UI values via system property.

The matching change to build system now writes these values as build
properties for recovery image. This allows us dropping the dependency on
Android.mk (as well as having more flexibility to do UI customization).

Also rename a few constant names, as the naming doesn't fully follow the
style guide (which reads "whose value is fixed for the duration of the
program").

Bug: 110380063
Test: Build and flash recovery image on taimen, which uses custom margin
      height. Check the UI and choose `Run graphics test`.
Change-Id: I2c50326123639cb36022f51b62cdeed925d77ba7
diff --git a/ui.h b/ui.h
index a1e18cc..e0fb13e 100644
--- a/ui.h
+++ b/ui.h
@@ -195,8 +195,8 @@
   };
 
   // The sensitivity when detecting a swipe.
-  const int kTouchLowThreshold;
-  const int kTouchHighThreshold;
+  const int touch_low_threshold_;
+  const int touch_high_threshold_;
 
   void OnKeyDetected(int key_code);
   void OnTouchDetected(int dx, int dy);