Make LockscreenCredential remember whether it has invalid chars am: 019ec7a6ff am: 0569316624 am: 2b99fb823e

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/2775376

Change-Id: Iaf926ec1ec826df29a0ff123482e99205690b39d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>