Add basic ability to login to an existing numberless account.

core/serialization/src/main/java/org/signal/core/util/serialization/ACISerializer.kt

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util.serialization
+
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.PrimitiveKind
+import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import org.signal.core.models.ServiceId.ACI
+
+class ACISerializer : KSerializer<ACI> {
+  override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("ACI", PrimitiveKind.STRING)
+
+  override fun deserialize(decoder: Decoder): ACI {
+    return ACI.parseOrThrow(decoder.decodeString())
+  }
+
+  override fun serialize(encoder: Encoder, value: ACI) {
+    encoder.encodeString(value.toString())
+  }
+}