Merge "Check for overflow in String16::append and String16::insert." into sc-dev am: eec890e173 am: 117b7cd3e9

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/14940186

Change-Id: I6572e3288efc7e3ac203c8688ea2fc08c8260bfc