binder: allow repeated invocations of initWithDriver

As long as the driver name is the same, calls to
ProcessState::initWithDriver() will succeed, returning the
already-created object.

Test: marlin

Change-Id: I87d5ca2a105becd8d4fa6fa85c00b879360229b3
Signed-off-by: Iliyan Malchev <malchev@google.com>