Always set ViewPager adapter immediately.

This appears to be an oversight and may be causing b/8270164.

cherry-pick of: https://googleplex-android-review.googlesource.com/285178
Change-Id: I840a79d28c55d478a1182059b057da092947720b
diff --git a/src/com/android/ex/photo/PhotoViewActivity.java b/src/com/android/ex/photo/PhotoViewActivity.java
index fad1b31..f626757 100644
--- a/src/com/android/ex/photo/PhotoViewActivity.java
+++ b/src/com/android/ex/photo/PhotoViewActivity.java
@@ -182,6 +182,7 @@
         final Resources resources = getResources();
         mRootView = findViewById(R.id.photo_activity_root_view);
         mViewPager = (PhotoViewPager) findViewById(R.id.photo_view_pager);
+        mViewPager.setAdapter(mAdapter);
         mViewPager.setOnPageChangeListener(this);
         mViewPager.setOnInterceptTouchListener(this);
         mViewPager.setPageMargin(resources.getDimensionPixelSize(R.dimen.photo_page_margin));