Standalone ANGLE checkouts do not contain //tools/android/md5sum, which
means the necessary BUILD.gn is also missing. That BUILD.gn is required
by build/config/android/internal_rules.gni which ANGLE depends on, but
doesn't use in standalone builds (since standalone ANGLE can't build
Android).

Standalone ANGLE GN generation is required by the Android source tree as
part of the automated Android.bp generation. Building Android
executables with the generated GN output is not required or supported.

This CL creates an empty //tools/android/md5sum/BUILD.gn to satisfy GN
as a workaround until //tools/android/md5sum is moved into //build.

Bug: chromium:1110408
Test: gn gen out/Android
Change-Id: Ia4b4fbf63b9ccecd9415784e64f646330b33224d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2335736Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>