Add a --copy-to-vendor-partition optional argument to both roll_aosp.sh
and generate_android_bp.py. When specified the root targets will be
generated with vendor: true and a relative install path to egl folder,
so that we can use ANGLE as the Android OpenGLES driver when the system
starts up.

Test: run roll_aosp.sh --copy-to-vendor-partion, build the Android image
with m -j72, load the image with the Android emulator and find all ANGLE
related *.so's are under the /vendor/lib/egl folder.

Bug: angleproject:5456
Change-Id: I38c64e8ea3ad5f5d0cc0b8cd77856f49109b23c7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2568907
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>