[startop] Add an app for testing startup performance

This CL includes a simple app for testing aspects of startup performance. It
will probably gain new activities as we focus on different parts in
particular. Currently it has an empty activity and an activity that focuses on
layout inflation.

Test: mma, adb install, adb shell start (see README.md for exact commands)
Change-Id: I4e98994f0c25ce61a89d13f86eda2320a1fdcafa