Add a performance test for launching DocumentsUI.

The test launches the DocumentsUI as picker, then waits until the
main thread idles, which guarantees that roots are loaded and UI
rendered.

It confirms, that the recent system cache improves cold start
performance by around 24% on my configuration (from 1685ms to 1357ms).

Bug: 27370274
Change-Id: I738202ea434a7bfe7080fc0994f636ef0e7847cd