Offer conversations in the system share sheet

Sharing from another app listed Element X once, with no way to pick a
conversation: the Direct Share row never showed any.

The app already publishes a long-lived, conversation-categorised shortcut per
room, which is most of what the sharing shortcuts API needs. The part that was
missing is the declaration that ties those shortcuts to the share intent: the
launcher activity had no `android.app.shortcuts` meta-data, so there was no
`<share-target>` for the system to match them against.

It now ships one, matching the activity that already receives ACTION_SEND, and
the shortcuts carry the category it names.

The system offers these from Android 10 onwards, where sharing shortcuts are
handled natively. Reaching further back would additionally need the
androidx.sharetarget library, which is not added here.

Part of #3835

app/src/main/AndroidManifest.xml

@@ -139,6 +139,12 @@
                 <data android:host="user" />
                 <data android:host="room" />
             </intent-filter>
+            <!--
+              Conversation shortcuts offered in the Direct Share row of the system share sheet.
+            -->
+            <meta-data
+                android:name="android.app.shortcuts"
+                android:resource="@xml/shortcuts" />
             <!-- Incoming share simple -->
             <intent-filter>
                 <action android:name="android.intent.action.SEND" />

app/src/main/res/xml/shortcuts.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Copyright (c) 2026 Element Creations Ltd.
+
+  SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+  Please see LICENSE files in the repository root for full details.
+-->
+<shortcuts xmlns:android="http://schemas.android.com/apk/res/android">
+    <!--
+      Lets the system offer the conversation shortcuts published by
+      DefaultNotificationConversationService in the Direct Share row of the share sheet.
+      The category must match SHARE_TARGET_CATEGORY in that class.
+    -->
+    <share-target android:targetClass="io.element.android.x.MainActivity">
+        <data android:mimeType="*/*" />
+        <category android:name="io.element.android.category.SHARE_TARGET" />
+    </share-target>
+</shortcuts>