Ensure "add a number" gets read appropriately.

The button style uses upper-case text to label the button.  This means
"add a number" is read as "A - D - D a number".
Setting the content description solves this (since the original string
is not all upper-cased).

Test: Manual with screen reader
Bug: 73557137
Change-Id: I630f9f328ba35c6b065c4dec27e8c6eff58e218b