Merge "Clarify docs for FLAG_SYSTEM" am: 40dcbcad28 am: b4ff7c066f am: 687923e2c7

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1426971

Change-Id: I8b6e9e6de24d6c6332a5a224d1375ebc63c08899