Merge "Implement String8|16::empty and String16::length" into main am: 9578c774a9 am: 80738a1e0b

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

Change-Id: Idc628dff47b00a0f151c94d011f5ebf18edd6fe4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>