Fix signing process for SELinux file_contexts

When not building locally and just using a target-files.zip from some
other build, it still tries to access the file_contexts from the out/
directory. This change instead looks at the unzipped target-files.zip
hierarchy to grab that information.

Bug: 9191141
Change-Id: I6ea12e82d6c6376fcada412314c5eefc97ff4853