Always mix in additional entropy into keymaster.

This makes AndroidKeyStore Cipher and KeyGenerator implementations mix
in additional entropy into keymaster's RNG regardless of whether they
were provided with a SecureRandom instance.

In practice, they are always provided with a SecureRandom instance.
However, to be safe, when no SecureRandom instance is provided the
code now uses a platform-default SecureRandom implementation.

Bug: 18088752
Change-Id: I85bca30d7bdc82c2a342094dcbe6044e48a63dca