Merge "Clarify docs for FLAG_SYSTEM" am: 40dcbcad28 am: b4ff7c066f am: 687923e2c7 am: 8ce81bcb12 am: 2046ae1522

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

Change-Id: I26725df92f2f114ca1a43c6471173aaf7d20630e