Reduce PackageInstaller I/O pressure.

When performing a restore during initial device setup, we could be
installing hundreds of packages.  Currently, we're writing all
metadata (including heavy icons) for every session mutation!  Because
we're holding the mSessions lock while writing all this heavy data,
we end up causing ANRs when apps call other PackageInstaller APIs.

This patch mitigates by moving the heavy icon data into separate
per-session PNG files, which we only persist when changed.

Bug: 17881962, 17567794
Change-Id: I4dee15d4a65a8eb65c381e6bb7477728b6cc30d2