Standardize screenshot naming scheme

Remove "global_" prefix across the screenshot code. Previously
names were mixed between "screenshot_" and "global_screenshot_"
which made finding files/elements confusing.

Bug: 191661294
Fix: 191661294
Test: manual -- no logic change, just renaming
Change-Id: I65ba13601660007ea38d08515adb5afecf39bf2e
12 files changed