Navigation stack native primitives (#139)

Adds support for stack navigation primitives (UINavigationViewController and Android fragment container with back button support)

android/src/main/java/com/swmansion/rnscreens/ScreenStackHeaderSubview.java

@@ -0,0 +1,77 @@
+package com.swmansion.rnscreens;
+
+import android.view.View;
+import android.view.ViewParent;
+
+import com.facebook.react.bridge.ReactContext;
+import com.facebook.react.uimanager.UIManagerModule;
+import com.facebook.react.views.view.ReactViewGroup;
+
+public class ScreenStackHeaderSubview extends ReactViewGroup {
+
+  public class Measurements {
+    public int width;
+    public int height;
+  }
+
+  public enum Type {
+    LEFT,
+    CENTER,
+    TITLE,
+    RIGHT
+  }
+
+  private int mReactWidth, mReactHeight;
+  private final UIManagerModule mUIManager;
+
+  @Override
+  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
+    if (MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY &&
+            MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY) {
+      // dimensions provided by react
+      mReactWidth = MeasureSpec.getSize(widthMeasureSpec);
+      mReactHeight = MeasureSpec.getSize(heightMeasureSpec);
+      ViewParent parent = getParent();
+      if (parent != null) {
+        forceLayout();
+        ((View) parent).requestLayout();
+      }
+    }
+    setMeasuredDimension(mReactWidth, mReactHeight);
+  }
+
+  @Override
+  protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
+    if (changed && (mType == Type.CENTER || mType == Type.TITLE)) {
+      Measurements measurements = new Measurements();
+      measurements.width = right - left;
+      if (mType == Type.CENTER) {
+        // if we want the view to be centered we need to account for the fact that right and left
+        // paddings may not be equal.
+        View parent = (View) getParent();
+        int parentWidth = parent.getWidth();
+        int rightPadding = parentWidth - right;
+        int leftPadding = left;
+        measurements.width = Math.max(0, parentWidth - 2 * Math.max(rightPadding, leftPadding));
+      }
+      measurements.height = bottom - top;
+      mUIManager.setViewLocalData(getId(), measurements);
+    }
+    super.onLayout(changed, left, top, right, bottom);
+  }
+
+  private Type mType = Type.RIGHT;
+
+  public ScreenStackHeaderSubview(ReactContext context) {
+    super(context);
+    mUIManager = context.getNativeModule(UIManagerModule.class);
+  }
+
+  public void setType(Type type) {
+    mType = type;
+  }
+
+  public Type getType() {
+    return mType;
+  }
+}