SF: Store transaction traces as a serialized blob

Reduce memory usage for transaction traces by storing
the data after serialization since this will pack the
messages tightly.

Test: presubmit
Test: awk '/Private_Dirty:/{ sum += $2 } END { print sum }' /proc/`pidof surfaceflinger`/smaps
Bug: 200284593
Fixes: 210078963
Change-Id: I6d6324264b112a08225bb4335b27fd19dd832d08