String16::remove - avoid overflow am: 4048e49956 am: 107f18cb61 am: 9be245c3f0 am: 64fb5012b8

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/11619802

Change-Id: I03057c052a24f205c78ee600178d45e0cd51a3c1