The definition _LIBCPP_ABI_UNSTABLE breaks std::string in Android AOSP
builds, starting with GetHelperExecutableDir(), but others fail later
if that's worked around. Instead, we'll disable that flag with a GN arg
for AOSP rolls.

Bug: angleproject:5263
Test: Build on AOSP and launch Cuttlefish
Change-Id: Ia4b293a5ce0b09206f4ed87a5a442e544e2021c1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2508529Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>