Check parameters in KeyGenerator.init.

KeyGenerator.init is supposed to check whether all provided parameters
are OK. This is because KeyGenerator.generateKey cannot throw checked
exceptions. This CL makes AndroidKeyStore KeyGenerator implementation
do just that. Unfortunately, keymaster/kestore doesn't provide a way
to check whether all the parameters are OK without actually generating
a key. Thus, this KeyGenerator does its best inside init method
(before Keymaster is called), and then surfaces any remaining issues
(flagged by Keymaster/keystore) as unchecked IllegalStateException.

Bug: 18088752
Change-Id: I9a04da880dcbe26c37f41d1477e41bdc74db04c9