Remove null warnings - Actually clean up server & client keys

[vteague]

This removes warnings about possible null return & dereference values, but only from the parts of the code that relate to client & server cryptographic keys.

Also does some other clean up of key storage etc, including removing the client signing key from Android backup storage.

[vteague]

This doesn't solve the problem that on iOS we need the keychain entitlement set up to run on a simulator.