add MANAGE_CREDENTIALS appop

This is part of the work to support
a credential management app on
unmanaged devices.

Bug: 179180345
Test: atest android.appsecurity.cts.AppOpsTest

Merged-In: Ifd87b5e38f341db4b34bf9f61480e29e49148846
Change-Id: Ifd87b5e38f341db4b34bf9f61480e29e49148846