Ignore /out_aar and check build_aar --build-dir

build_aar.py creates a per-ABI GN output directory at
<build_dir>/<arch>, and Siso's Starlark configuration in
build/config/siso/platform.star hardcodes a two-level relative path
(../../) from the Ninja output directory to the source root. Passing a
nested path such as out/android-archive to --build-dir places the GN
output directory three levels deep under src/, breaking Siso remote
execution (see crrev.com/c/8642070 and crrev.com/c/8795469).

Check that --build-dir is a direct subdirectory of SRC_DIR in
build_aar.py, update the --build-dir help string, and add /out_aar
(DEFAULT_BUILD_DIR in build_aar.py, now used by build_android_archive in
crrev.com/c/8795469) to .gitignore.

Bug: webrtc:575920311
Change-Id: I94bf268c8cb99c163ea5e6001ed40fb3b926ba32
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/515082
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#49251}