Always build absolute paths the same way

( cherry-picked from 8decf95ffeffcc8aa14aa743ae398bc38d54b0ac )

Fix a bug where output paths like:


triggered bad logic to detect relative paths and turned the path into


Fix this by moving absolute path detection logic into a common helper
function and calling it consistently.

Bug: 29091703
Test: unittests, angler-eng builds with this change, manual testing on
      windows demonstrates issue is fixed

Change-Id: I2431f3dbcc389902d9f133197ec0026c20e75a8c
5 files changed