Merge branch '2229-mailbox-client' into 'master'

.../src/main/java/org/briarproject/bramble/api/Supplier.java

...g/briarproject/bramble/api/mailbox/MailboxProperties.java

...project/bramble/api/mailbox/MailboxUpdateWithMailbox.java

...rc/test/java/org/briarproject/bramble/test/TestUtils.java

...ava/org/briarproject/bramble/client/ClientHelperImpl.java

...c/main/java/org/briarproject/bramble/mailbox/ApiCall.java

...org/briarproject/bramble/mailbox/ConnectivityChecker.java

...briarproject/bramble/mailbox/ConnectivityCheckerImpl.java

...ct/bramble/mailbox/ContactMailboxConnectivityChecker.java

...va/org/briarproject/bramble/mailbox/MailboxApiCaller.java

...rg/briarproject/bramble/mailbox/MailboxApiCallerImpl.java

...java/org/briarproject/bramble/mailbox/MailboxApiImpl.java

.../briarproject/bramble/mailbox/MailboxPairingTaskImpl.java

...arproject/bramble/mailbox/MailboxSettingsManagerImpl.java

...riarproject/bramble/mailbox/MailboxUpdateManagerImpl.java

...roject/bramble/mailbox/OwnMailboxConnectivityChecker.java

.../java/org/briarproject/bramble/mailbox/SimpleApiCall.java

...org/briarproject/bramble/client/ClientHelperImplTest.java

...rproject/bramble/mailbox/ConnectivityCheckerImplTest.java

...ramble/mailbox/ContactMailboxConnectivityCheckerTest.java