Keystore: Use security levels

In anticipation of the availability of Keymaster implementations with
multiple security levels this patch adds the additional
keystore flags FLAG_SOFTWARE and FLAG_STROGBOX.

Also, the IKeystore method addRngEntropy got a new flags parameter
for the caller to express which implementation shall be awarded the
precious entropy.

Test: Keystore CTS tests
Bug: 63931634
Change-Id: I4a4eafbdbe1290f0c7bd2bfa2ce3e5fbb06c2dd8