Log outgoing transactions/replies over 300kB.

There's some occurences of a transaction just under 1 MB into
system_server, which causes system_server to reject other transactions,
and sometimes throw DeadSystemException.

Until we have better backend infrastructure in place, at least log the
source and some data of really large transactions, which might give us
some sort of clue.

Bug: 198380036
Bug: 213349547
Test: N/A
Change-Id: Ifccf4a2dea32cd43421b61d3003d2a2bdc86ee23