freezer: don't allocate stats on unfreeze

Unfreeze operations could happen quite frequently. If they're
synchronous with ApplicationManager main thread they must also be
lightweight, so avoid any unnecessary allocations and reuse message
fields for log data.

Bug: 151187166
Test: installed on a flame and verified log messages are correct.
Change-Id: Ie5e22cdc5538277dbf19b5a5fffe505ca07a8083