adb: search for a maximum of 16 emulators.

Android Wear has unfortunately been using port 5601 for years, which
falls into the range of ports we were previously polling for Android
emulators. Reduce the maximum number of emulators we can support so
that 5601 no longer falls within our range.

Bug: http://b/26468076
Change-Id: I931809cfa412122f4781eebe0164facab12c95f0