Merge "Implement String8|16::empty and String16::length" into main am: 9578c774a9 am: 80738a1e0b am: 3cca1adac9 am: 51115c6d18 am: 2d0a56ca4e

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

Change-Id: Ib0cee886bd574ea8bac10d3f3a499f6520f158a6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>