Adds Text to the common module.

UserSwitcherActivity refactor: CL 2/7

Following the precedent set by Icon and ContentDescription, Text is a
sealed class that can represent a string that is either already loaded
into memory or is referenced to using a resource ID.

This CL also includes the TextViewBinder.

Bug: 243844359
Test: Manually tested as part of CL 7/7 in the chain
Change-Id: I85bc6866fff9d37e573d4cdeaa74da8efbb35246