Add flag so that users' typed text isn't used for personalisation

[CDRussell]

Task/Issue URL: https://app.asana.com/0/search/587554197702895/567594137922103
Tech Design URL:
CC:

Description:
By default, any text the user types into the app can be used for personalisation by whichever keyboard is in use.

This PR will add the non-personalisation flag to request that doesn't happen for text typed in our app. Note, this isn't a guarantee and is just a request; the keyboard app can do whatever it wants still.

There are two places to apply the flag; one is on the omnibar EditText as that is a native component, and another is for keyboards spawned through the WebView.

The flag is only applicable for devices running Oreo and upwards.

Discussion
Do you think this needs to be a user-preference? It feels safe enough to me to always have it enabled.

Steps to test this PR:

1. Launch the app
2. Click on the omnibar
3. If using standard Android GBoard, ensure you are in keyboard incognito mode. This can be verified by the very faint spy watermark.
4. If using another keyboard, then it probably isn't respected anyway, so you're unlikely to notice any change.
5. Load a webpage, like our SERP, and start typing content INSIDE the WebView itself and ensure you see the same incognito watermark.

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Internal references:

Software Engineering Expectations
Technical Design Template

[CDRussell]

Example of incognito watermark

[brindy]

LGTM