generate_android_bp.py prepends "angle_" to target names to avoid
conflicts with other Android projects. This can sometimes generate
conflicts when there are two targets such as "angle_vulkan_headers"
and "vulkan_headers".

This patch turns the gn path+target into the blueprint target name.

BUG= b/155396154

Change-Id: I7a709013969ae8e312a781a2fd3c1ec530fca430
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2173833
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>