msglist: Update icon of "Mark all messages as read" button

[chrisbobbe]

We've decided to change the icon for the "Mark all messages as read" button at the bottom of the message list. Here's what it looks like now:

It should instead match what we use in web:

The source should be this SVG in Figma.

Discussion: https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/mark-as-read.20icon/near/2092108

[ashwanisng]

Can I Work on this?
cc: @chrisbobbe @gnprice

[chrisbobbe]

This is being fixed in #1274.