Keystore2: Batching listing of key entries

Add methods to the keystore2 interface for the following:
* Getting the total number of entries in a given domain and namespace.
* Get a batch of key descriptors starting past a given value.

See the keystore2 change for detailed description.

Bug: 222287335
Test: See keystore2 change.
Change-Id: I449fcebf032af77374c4eed980966dec6878e00c
2 files changed