Initial commit of InputMethodStressTest.

This CL added a simple test of showing and hiding IME for 100 times.

TODOs:
1. Add more complicated flows (such as notification).
2. Check IME visibility in several points (WindowManager dump,
   SurfaceFlinger dump, etc), not just the inset that the app received.

Bug: 195468725
Test: atest InputMethodStressTest
Change-Id: I1959225a7396d17750d799e11c42eb1899fe5bb9