Change update detection to reduce load time.

Reduces load time if extraction was already made. It appeared that
new ZipFile was really slow because it's preparing much things as
soon as it's instanciated.

The new criteria consist of the last modified time of the apk plus
the crc of the apk's central directory, last modified time should
be enough for nearly all modifications and the crc is here to try
to handle an OTA mixing with dates.

The transition from old criteria to new should be good: since there
will be no stored values they would be detected as a new installation.

Change-Id: Id390b77b03d794b8b7feb91eb0daae1126c6d691