[Android] Move abi_target from gn to gni

Move the abi_target setting logic into abi.gni and rename it to
android_abi_target, so that other gn file can easily access it.

Also provide a util to use the android_abi_target for copying
libc++_shared.so into the APK.

Bug: internal b/183981020
Test: Build and run on device
Change-Id: I9e4dc56583f328b03b2484b8844ce68385c0f9c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2812428
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871195}