libbinder: RPC disallow nested oneway transactions

Previously, nested transactions were accidentally allowed while
processing oneway transactions. This changes things so that nested
transactions are only explicitly allowed when a synchronous transaction
is being processed (like how kernel binder is).

Future considerations: this CL makes it more explicit that we allow
refcount transactions as part of nested transactions. This is okay
because 'drainCommands' will process these, but there might be some
delay. We could make refcount behavior nicer if we always preferred
using an active threadpool (if one is available) to process them.

Bug: 167966510
Test: binderRpcTest
Change-Id: Iaeb472896654ff4bcd75b20394f8f3230febaabf