Batch fetching of key descriptors from Keystore

Change interaction with Keystore2 in the following manner:
* Return an enumerator over the entries in Keystore2 rather than
  attempting to get all of them into one single data structure.
* Use a new Keystore2 method for getting the count of entries
  rather than count the size of the array returned.

The enumerator reads a batch of key descriptors from Keystore2.
Once the batch has been exhausted, the enumerator added asks
Keystore2 for the next batch of keys starting with the last
alias it has processed, until it receives an empty array.

Bug: 222287335
Test: atest KeystoreTests
Change-Id: I309b3188df998825557a3c5e6d777b1c0807a924