prefer releasetools.py from target_files zip

If the target_files zip for the target build contains a
META/releasetools.py (which it has since Nov 2013), prefer that over
using a releasetools.py from the local client.

Explicitly specifying the device-specific extensions path via
command-line options takes priority over both of the above mechanisms.

Change-Id: Ia068b0e2e06ede7da89ebe4315cdec592eb8995e