Some improvements on strings

-- core --
* Replace "your tablet/phone" with "your Nexus"
    in shutdown dialog, to make it consistent with
    our reboot dialog.
* Delete reboot_system strings which are already dead.

-- SystemUI --
* Replace "Sleep" tile label with "Power button"
    which makes more sense since longpress the
    tile opens the power menu.

一些strings的改进

-- core --
* 替换关机对话框中的"您的平板电脑/手机"为"你的Nexus",
    以保持和重启对话框的一致性.
* 删除reboot_system的strings, 他们已经死了...

-- SystemUI --
* 替换"锁屏"磁贴标签为"电源按钮", 这样更合理一些
    因为长按这个磁贴会打开电源菜单.