dock:: fix wrong theme icon is getton.

return icon name instead of the absolute path of icon. the front end
will get the icon by itself.

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/e95ad265026843e5a8ff7d840a9c7816/
Change-Id: I823e5e5e40fac656c3f711f4f58b61f5dad86730