Remove LocalKeyStore's dependency on Account to reduce coupling
Remove LocalKeyStore's dependency on K9.app
Don't delete certs. on account creation, & assure that only one instance of LocalKeyStore is created.
Eliminate the need to pass a context to LocalKeyStore.getInstance

Instead, have K9.onCreate initialize the location of the key
store file (similar to what is done with
BinaryTempFileBody.setTempDirectory).

Also, LocalKeyStore.getInstance has been changed so that it
no longer needs to be synchronized.