Refactor and improve attachment deduping logic.

app/src/main/java/org/thoughtcrime/securesms/attachments/AttachmentId.kt

@@ -3,13 +3,14 @@ package org.thoughtcrime.securesms.attachments
 import android.os.Parcelable
 import com.fasterxml.jackson.annotation.JsonProperty
 import kotlinx.parcelize.Parcelize
+import org.signal.core.util.DatabaseId
 
 @Parcelize
 data class AttachmentId(
   @JsonProperty("rowId")
   @JvmField
   val id: Long
-) : Parcelable {
+) : Parcelable, DatabaseId {
 
   val isValid: Boolean
     get() = id >= 0
@@ -17,4 +18,8 @@ data class AttachmentId(
   override fun toString(): String {
     return "AttachmentId::$id"
   }
+
+  override fun serialize(): String {
+    return id.toString()
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/attachments/PointerAttachment.kt

@@ -2,6 +2,7 @@ package org.thoughtcrime.securesms.attachments
 
 import android.net.Uri
 import android.os.Parcel
+import androidx.annotation.VisibleForTesting
 import org.signal.core.util.Base64.encodeWithPadding
 import org.thoughtcrime.securesms.blurhash.BlurHash
 import org.thoughtcrime.securesms.database.AttachmentTable
@@ -14,7 +15,8 @@ import org.whispersystems.signalservice.internal.push.DataMessage
 import java.util.Optional
 
 class PointerAttachment : Attachment {
-  private constructor(
+  @VisibleForTesting
+  constructor(
     contentType: String,
     transferState: Int,
     size: Long,