The buildtools/ directory is not included in the AOSP version of ANGLE
and is unnecessary, so update generate_android_bp.py to filter out that
include directory.

Bug: b/187342779
Change-Id: I3a2d0506ac53f7d369597c797fffb211ffde41f1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2995382Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>