Update User Agent to Add Metamask

**Describe the usability problem**
Many Apps with in-App Browser include some kind of identification string in their user agent.
In that way, developers can react to different browsers.

Could you please add something like "MetamaskBrowser" or anything similar to your user agent?

Mozilla/5.0 (Linux; Android 10; Android SDK built for x86 Build/OSM1.180201.023) **MetaMaskBrowser** AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.92 Mobile Safari/537.36'

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**User impact | Usage frequency | Feasibility scores**
_How important is this to the user, what the user wants to accomplish | Daily, weekly, monthly | Small, medium, large_

The issue does not name any files, tests, or entry points. Start by locating where the mobile browser user agent is assembled for Android and iOS, then verify how the added MetaMask identifier appears in the reported user-agent string and test the affected browser flows.

Written by the indexing model from the issue text.