When replying to a message, don't use an earlier timestamp.

...src/net/sf/briar/android/contact/ContactListActivity.java

.../sf/briar/android/contact/ReadPrivateMessageActivity.java

...sf/briar/android/contact/WritePrivateMessageActivity.java

...rc/net/sf/briar/android/groups/ReadGroupPostActivity.java

...c/net/sf/briar/android/groups/WriteGroupPostActivity.java

briar-api/src/net/sf/briar/api/messaging/MessageFactory.java

...r-core/src/net/sf/briar/messaging/MessageFactoryImpl.java

briar-tests/src/net/sf/briar/ProtocolIntegrationTest.java

briar-tests/src/net/sf/briar/messaging/ConstantsTest.java

...ar/messaging/simplex/SimplexMessagingIntegrationTest.java