Sharesheet content preview - adding text support only

Adding initial support for a content preview section within
the ChooserActivity. File/Image support is pending.

Bug: 120419296
Test: 'atest ChooserActivityTest' as well as visual inspection
of sharing from multiple google apps like Chrome

Change-Id: Iee4746940fb8ddd4f0a54a0bf7ef485be2eab30d