Add --build_dir arg to build_aar.py
Add --build_dir arg to build_aar.py

This will enable incremental build for aar

BUG=None

Review-Url: https://codereview.webrtc.org/3008973002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19689}

TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc