Add message delivery state icons from Android

Converted with https://gitlab.com/AlessandroLucchet/VectorDrawable2Svg/-/blob/d401706209b26f7be445a15fed1b217164e48802/VectorDrawable2Svg.py

Sources:
* https://code.briarproject.org/briar/briar/-/blob/4e5f2e31df85cb6d56559bb99bc921d322f6a1af/briar-android/src/main/res/drawable/message_delivered.xml
* https://code.briarproject.org/briar/briar/-/blob/4e5f2e31df85cb6d56559bb99bc921d322f6a1af/briar-android/src/main/res/drawable/message_sent.xml
* https://code.briarproject.org/briar/briar/-/blob/4e5f2e31df85cb6d56559bb99bc921d322f6a1af/briar-android/src/main/res/drawable/message_stored.xml

Related to #52.

briar-gtk/data/icons/hicolor/scalable/status/message_delivered.svg

+<?xml version="1.0" ?>
+<svg xmlns="http://www.w3.org/2000/svg" width="24.0" height="24.0" viewBox="0 0 24.0 24.0">
+  <path d="M18,7l-1.41,-1.41 -6.34,6.34 1.41,1.41L18,7zm4.24,-1.41L11.66,16.17 7.48,12l-1.41,1.41L11.66,19l12,-12 -1.42,-1.41zM0.41,13.41L6,19l1.41,-1.41L1.83,12 0.41,13.41z" fill="#FFFFFFFF"/>
+</svg>

briar-gtk/data/icons/hicolor/scalable/status/message_sent.svg

+<?xml version="1.0" ?>
+<svg xmlns="http://www.w3.org/2000/svg" width="24.0" height="24.0" viewBox="0 0 24.0 24.0">
+  <path d="M9,16.17L4.83,12l-1.42,1.41L9,19 21,7l-1.41,-1.41z" fill="#FFFFFFFF"/>
+</svg>

briar-gtk/data/icons/hicolor/scalable/status/message_stored.svg

+<?xml version="1.0" ?>
+<svg xmlns="http://www.w3.org/2000/svg" width="24.0" height="24.0" viewBox="0 0 24.0 24.0">
+  <path d="M11.99,2C6.47,2 2,6.48 2,12s4.47,10 9.99,10C17.52,22 22,17.52 22,12S17.52,2 11.99,2zM12,20c-4.42,0 -8,-3.58 -8,-8s3.58,-8 8,-8 8,3.58 8,8 -3.58,8 -8,8zM12.5,7H11v6l5.25,3.15 0.75,-1.23 -4.5,-2.67z" fill="#FFFFFFFF"/>
+</svg>