cryptfs: check for errors in create_encrypted_random_key()

When generating the key and salt we weren't checking for an error
opening or reading from /dev/urandom.  Switch to the helper function
ReadRandomBytes() and start checking for errors.

Test: Booted device with FDE.  As a extra sanity check I also
      temporarily added log messages that dump the key and salt,
      and I verified they still appear random.
Change-Id: I01ccee4f1f9910bf9508c8f02a918157393b0e68
1 file changed