Add a content description to the back button

This is mostly to provide a way to identify it through tests
that doesn't rely on ids. When using Q-style RROs on the toolbar,
the id is changed and breaks tests. We use these RROs for RTL
support.

Fixes: 158522411
Bug: 154241156
Test: Manually
Change-Id: Ib8d0b21005b64cefde348fae12390eb1e3ab2308
3 files changed