android/system/monkey: use 0 physical keys as default.
Created by: balsini
It may happen that, while launching tests on platforms withouth physical keys (e.g. Hikey), monkey fails returning
** SYS_KEYS has no physical keys but with factor 2.0%.
becasue of its default configuration. This is solved by explicitly defining the --pct_syskeys parameters, 0 as default value.