Android builds are failing due to .git/HEAD being listed in the
Android.bp, which generate_android_bp.py is supposed to remove. The
problem is that the blacklist contains '//third_party/angle/.git/HEAD'
rather than '//.git/HEAD', due to generating the GN files from
chromium/src/ rather than angle/. This CL cleans this up.

Bug: b/160727922
Test: Manual script execution and building in AOSP
Change-Id: Ia1682605097791d20b28686f3043ddc08d49ca52
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2310960Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>