Merge "Implement String8|16::empty and String16::length" into main am: 9578c774a9

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2705822

Change-Id: If14f11f1ec862554e698f443195751b8a997eba9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>