The Extensions SDK is what makes features like Photo Picker possible on older Android versions.
Project Mainline is a way for Google to deliver updates to some critical system components through the Google Play Services framework and the Google Play Store. Each Mainline module is delivered as either an APK file, an APEX file, or an APK-in-APEX. When a Mainline module is being updated, the user sees a “Google Play System Update” (GPSU) notification on their…
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