Enable opening mailto URLs under Android

* doc/emacs/android.texi (Android Startup): Mention how mailto
URLs are treated by the emacsclient wrapper.

* java/AndroidManifest.xml.in: Register `mailto' scheme filters
for EmacsOpenActivity.

* java/org/gnu/emacs/EmacsOpenActivity.java (startEmacsClient):
Extract code that starts Emacs when it isn't already running,
and take a list of arguments rather than a single file name.
(onCreate): If the scheme is `mailto', escape the URI and call
`message-mailto'.

java/AndroidManifest.xml.in

@@ -122,6 +122,21 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>. -->
         <category android:name="android.intent.category.BROWSABLE"/>
         <data android:scheme="org-protocol"/>
       </intent-filter>
+
+      <!-- And also mailto links.  -->
+
+      <intent-filter>
+        <action android:name="android.intent.action.VIEW"/>
+        <category android:name="android.intent.category.DEFAULT"/>
+        <category android:name="android.intent.category.BROWSABLE"/>
+        <data android:scheme="mailto"/>
+      </intent-filter>
+
+      <intent-filter>
+        <action android:name="android.intent.action.SENDTO"/>
+        <data android:scheme="mailto"/>
+        <category android:name="android.intent.category.DEFAULT"/>
+      </intent-filter>
     </activity>
 
     <activity android:name="org.gnu.emacs.EmacsMultitaskActivity"