Fix IndexOutOfBoundsError in ScreenshotShelfView

We were calling index 0 without checking that the list was nonempty,
causing the screenshots UI to crash. Conditions:
1. Device is in three-button mode
2. App is in fullscreen
3. Screenshot is taken
4. User touches the screen.

Test: manual (100% repro as above before change, fixed after)
Bug: 444352245
Fix: 444352245
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:243b8ec1301ec405b12772727926e5141766c367
Merged-In: I6b81fe69fc46a7b42a4f26f48f37523807cce7e9
Change-Id: I6b81fe69fc46a7b42a4f26f48f37523807cce7e9