Merge "Add String16::c_str and start using it." into main am: 067dbcc1 am: 31d682b3

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2704555



Change-Id: I2482e45ffee5f2c9752ac7289e347013a38ea724
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>