releasetools: Use pattern filter when unzipping.

When building BBOTAs, it only needs *some* unzipped entries in the given
target_files zip(s). In particular, it needs 'IMAGES/*', 'META/*',
'RADIO/*'. (It also reads 'SYSTEM/build.prop' and 'OTA/bin/updater', but
directly from the zip file.)

This CL specifies the entries to unzip. It saves the I/O cost, as well as
the temporary storage.

Test: ota_from_target_files.py gives the same package w/ and w/o the CL.
Test: check_target_files_signatures.py still works.
Change-Id: I728428aa0e138879e49f9efbdb46a85892fc7038