clean up conversation history after swtiching to a different conversation

src/main/java/eu/siacs/conversations/Config.java

@@ -19,6 +19,7 @@ public final class Config {
 	public static final int MESSAGE_MERGE_WINDOW = 20;
 
 	public static final int PAGE_SIZE = 50;
+	public static final int MAX_NUM_PAGES = 3;
 
 	public static final int  PROGRESS_UI_UPDATE_INTERVAL = 750;
 

src/main/java/eu/siacs/conversations/entities/Conversation.java

@@ -129,6 +129,16 @@ public class Conversation extends AbstractEntity {
 		}
 	}
 
+	public void trim() {
+		synchronized (this.messages) {
+			final int size = messages.size();
+			final int maxsize = Config.PAGE_SIZE * Config.MAX_NUM_PAGES;
+			if (size > maxsize) {
+				this.messages.subList(0, size - maxsize).clear();
+			}
+		}
+	}
+
 	public void findUnsentMessagesWithOtrEncryption(OnMessageFound onMessageFound) {
 		synchronized (this.messages) {
 			for (Message message : this.messages) {
@@ -265,10 +275,6 @@ public class Conversation extends AbstractEntity {
 		}
 	}
 
-	public String getProfilePhotoString() {
-		return this.getContact().getProfilePhoto();
-	}
-
 	public String getAccountUuid() {
 		return this.accountUuid;
 	}

src/main/java/eu/siacs/conversations/ui/ConversationFragment.java

@@ -554,6 +554,7 @@ public class ConversationFragment extends Fragment {
 		}
 		if (this.conversation != null) {
 			this.conversation.setNextMessage(mEditMessage.getText().toString());
+			this.conversation.trim();
 		}
 		this.activity = (ConversationActivity) getActivity();
 		this.conversation = conversation;