[Cashtab] Only show extension install notification for webapp

Summary: This was showing up in the android app, make sure it's web only

Test Plan: npm test, ./start-android.sh with a tablet connected and DO NOT see the ext msg, in webapp we do if it is not installed (check incognito)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18984