Adds a getter for IllustrationPreference content description.

Needed for building accessibility node collection info for a list that
contains these preferences.

Bug: 318607873
Test: atest IllustrationPreferenceTest
Flag: NONE the change in topic using this getter is flagged
Change-Id: I15745465f148493e148c7489bffd4d780a6e2a34
2 files changed