libbinder: RPC process oneway w/ 'tail call'

When draining oneway commands (which must be serialized), we do a
recursive call to process a transaction. However, this wouldn't even be
considered to be a tailcall because of the complex destructors which
need to run. So, instead we work around this w/ goto to the beginning of
the function.

The alternative here (to a 'goto') to consider is creating a more
complex return type to processTransactInternal which would convince
processTransact to re-issue the command. Though, this would be a
somewhat larger refactor.

Fixes: 190638569
Test: binderRpcTest (OnewayStressTest repeatedly on device doesn't fail
    for several minutes - failed without this)
Change-Id: I9fbc75941452348e498849d5d59130487ef6cc44