Add more examples of generating/importing keys to Javadocs.

Based on developer feedback, this updates Android Keystore Javadocs
with more examples of generating and importing keys of various
algorithms. This also clarifies that key use authorizations apply to
secret and private key and do no apply to public keys.

Bug: 23102874
Change-Id: If0dc20fda4836fd23b9cd9c92490a04e71b19fc0