Make AuthorizedKeysDecoder thread-safe

Make AuthorizedKeysDecoder thread-safe

AuthorizedKeysDecoder is mutating state of a single instance, which is
counter-intuitive and leads to the last decoded key to be left present
in the instance.

Fix this by making the deconding method static and keeping the instance
an internal thing, closing the leak and making the decoding process
inherently thread-safe.

Change-Id: I000b100773d206aee6b0e4f634f7d46a8b56544e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>