libbinder: auto-choose between binder/vndbinder.

Right now, a huge amount of client code has to manually
specify initWithDriver("/dev/vndbinder"). This is kind
of silly because every vendor process is not allowed to
talk to /dev/binder, and system processes are not allowed
to talk to /dev/vndbinder. On legacy devices,
__ANDROID_VNDK__ will not be set, so they will continue
to talk to /dev/binder.

Bug: 62956284
Test: boot Pixel 2, check for errors
Change-Id: Ic0d9da6d9b451aa8b1e62f24487d19478bff3d97