Reduce footprint of Signature from ~7000 to ~1448

Signature had lazy initialization of the mString member when needed, but
it would stick around forever when initialized. Each package had one or
more Signatures that would be ~7000 bytes each. At a couple hundred
packages, that's over 1.3MB just for signatures.

Whenever packages.xml was written out, it would write the Signature for
each program as well which happens at boot thereby initializing the
mString member pretty much immediately.

Change-Id: Idb882ffeca2861b3e87437fc83f001710d6c0441