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

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

Change-Id: I8706d87f07629ff429417eedd97d441593cb2a4a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>