Experience upgrade splash screen.

Behaves similarly to the DatabaseUpgradeActivity. You have a
static list of ExperienceUpgrade models that include a "trigger"
version, where when a user upgrades through it a notification
will appear, and there will be a splash explanation screen.

Right now the splash screens are basic and not too configurable,
but that can be reworked as upgrades demand.

Closes #4151

src/org/thoughtcrime/securesms/IntroPagerAdapter.java

@@ -0,0 +1,38 @@
+package org.thoughtcrime.securesms;
+
+import android.support.v4.app.Fragment;
+import android.support.v4.app.FragmentManager;
+import android.support.v4.app.FragmentStatePagerAdapter;
+
+import java.util.List;
+
+public class IntroPagerAdapter extends FragmentStatePagerAdapter {
+
+  public static class IntroPage {
+    final int      backgroundColor;
+    final Fragment fragment;
+
+    public IntroPage(int backgroundColor, Fragment fragment) {
+      this.backgroundColor = backgroundColor;
+      this.fragment        = fragment;
+    }
+  }
+
+  private List<IntroPage> pages;
+
+  public IntroPagerAdapter(FragmentManager fm, List<IntroPage> pages) {
+    super(fm);
+    this.pages = pages;
+  }
+
+  @Override
+  public Fragment getItem(int i) {
+    IntroPage page = pages.get(i);
+    return page.fragment;
+  }
+
+  @Override
+  public int getCount() {
+    return pages.size();
+  }
+}