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

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

Change-Id: I67383aee64e52ea9f1b3c68801bd112ea74c4440
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>