clean_header: Run outside of $ANDROID_BUILD_TOP

Enable the use case where we run clean_header.py from outside of
$ANDROID_BUILD_TOP. Previously, this script required the current working
directory to be under $ANDROID_BUILD_TOP. Running it from a different
directory resulted in the following error message:

 clean_header.py: error: Not in android tree pointed at by ANDROID_BUILD_TOP (....)

(cherry picked from commit d12d6f67bca790696679db46cdc306eba3122e7c)

Bug: 128420573
Change-Id: If07b0345401f5dd35b41876a3838209595bf8ab1
Merged-In: I48210ea1a0033228a9aaa4124d28247b07cee6d4