Mark App Handle/header application name as important for a11y.

We want user to be able to focus on it so A11y services such as Talkback
can speak out the application name.

Bug: 329210454
Test: Manual. Talkback, open header/handle menu, focus on app name -
hear "Settings"
Flag: EXEMPT bugfix

Change-Id: I53452306b359720fa00a43ef07eb86598361a841