SF: Fix transaction trace merging

As entries get purged from the ring buffer, they need to be
merged in order to update the starting state. The tracing logic
tried to update the state directly from proto to be more
efficient. But this introduced slight changes in behavior.

Fix by reusing layerstate merge logic.

Test: atest TransactionProtoParserTest
Bug: 200284593
Change-Id: I8bfcf23c43fa89f3e5c4e899c5c8942d098bbe7f