Collect source_root in metrics.

Collecting source_root (ANDROID_BUILD_TOP env) to infer which branch users are using the tool
on.

Test: atest
Test: m adevice && adevice update -- check logs for android_build_top
Change-Id: If42f3f5ffb6fe0332c37c317435c4d53b5f1400e
6 files changed