package: Add some 'const' and prefix members

Change-Id: Id177c99ba8b91586cf61457ab2ef084af9c7f490
package: ZipOutputStream: prefix members

Change-Id: I9cc21ba87484560ec0db5424800dd67f80e00540
Split ZipPackageFolder::saveChild into two functions

And make them static. Probably would be better to kill ContentInfo and
add saveChild as pure virtual into ZipPackageEntry, from which are both
ZipPackageFolder and ZipPackageStream inheriting.

This will also create a bit more sensible call graph when profiling.

Change-Id: If8151332cfa6359e8736c912b7a5633a9162ab36
ZipOutputStream: No need to explicitly set default values

Change-Id: Ia3a20e0692e658d847e1b825fc97e32ac8a1854b