Restrict scanning port range, not emulator count

Our servers can comfortably run more than 16 emulators, and this is
much easier than messing with hypervisors or docker port allocation.

Assumes you know how to avoid clashes with Google Wear's default port.

Test: manual - still registers emulators.

Change-Id: I47cfd28725a550de0bd77fd11fcd94cdd11d2cc2