Add target for angle_white_box_perftests to DevSetup.md Android build
instructions. Generously comment the test source to clarify various
tests and how to run them.

Bug: angleproject:3136
Change-Id: Ie4f0ec6951d354ebac861f727c9d550a93ad48c9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1577740Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>