Compute better default scroll and zoom

This patch gives 2row keys a width of 31.25dp at default zoom (twice
for the 3row layout), and scrolls to the center of the keyboard.

android/src/com/levien/synthesizer/android/widgets/keyboard/KeyboardView.java

@@ -48,9 +48,9 @@ public class KeyboardView extends View {
     paint_.setTextAlign(Paint.Align.CENTER);
     strokeWidth_ = 1.0f * density;
     paint_.setStrokeWidth(strokeWidth_);
-    keyboardSpec_ = KeyboardSpec.make2Row();
     offset_ = 0.0f;
     zoom_ = 1.0f;
+    setKeyboardSpec(KeyboardSpec.make2Row());
   }
 
   public void setKeyboardSpec(KeyboardSpec keyboardSpec) {

android/src/com/levien/synthesizer/android/widgets/keyboard/ScrollStripView.java

@@ -35,13 +35,22 @@ public class ScrollStripView extends View {
     paint_.setColor(Color.GRAY);
     touchDown_ = new boolean[2];
     touchx_ = new float[2];
-    offset_ = -2000.0f;  // TODO: make more systematic
-    zoom_ = 2.5f;
+    zoom_ = -1f;  // placeholder value to be replaced in onLayout
   }
 
   public void bindKeyboard(KeyboardView kv) {
     keyboardView_ = kv;
-    kv.setScrollZoom(offset_, zoom_);
+  }
+
+  @Override
+  public void onLayout(boolean changed, int left, int top, int right, int bottom) {
+    if (zoom_ < 0) {
+      float density = getResources().getDisplayMetrics().density;
+      int width = right - left;
+      zoom_ = 3000f * density / width;
+      offset_ = -width * 0.5f * (zoom_ - 1);
+      keyboardView_.setScrollZoom(offset_, zoom_);
+    }
   }
 
   @Override