minadbd: rename adb_server_main to minadbd_main.

adb_server_main in adb refers to the adb server on the host, not adbd.
Since there doesn't seem to be a good reason to reuse the declaration
from adb's headers, give minadbd a main function of its own.

Change-Id: I748f1a6822dc14c726cb73ef3b533c57a6615608