binder: add getDriverName()

Ass ProcessState::getDriverName() to retrieve the /dev node that
libbinder is initialized with in this process.  This can be used
elsewhere to add debug code that is enabled for only one binder domain
or another.

Test: sailfish

Change-Id: I3ea9f0dcc97204508791bdeb70f790980be278a5
Signed-off-by: Iliyan Malchev <malchev@google.com>