8844cf7b8 triggered a regression where a new RPC wouldn't cause the
channel to exit idle mode, if an RPC was still progressing on an old
transport. This was already possible previously, but was racy.
8844cf7b8 made it less racy and more obvious.
The two added `exitIdleMode()` calls in this commit are companions to
those in `enterIdleMode()`, which detect whether the channel should
immediately exit idle mode.
Noticed in cl/635819804.