feat(app,android): a new icon, and a launcher label that fits

The icon follows proton-drive-linux's logo: a violet gradient tile with
a darker lower edge, and a white play mark with a soft shadow, cut into
the blocks a revision is fetched in. The SVG, the window PNG and the
Windows .ico are rendered from one source.

On Android it is an adaptive icon with a monochrome layer for themed
icons, and the notification glyph is the same mark. The app is named
"Proton Stream"; the launcher shows "Stream", since "proton-stream" was
cut to "proton-st…".

android/app/src/main/res/mipmap-anydpi/ic_launcher.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+    <background android:drawable="@drawable/ic_launcher_background" />
+    <foreground android:drawable="@drawable/ic_launcher_foreground" />
+    <monochrome android:drawable="@drawable/ic_launcher_monochrome" />
+</adaptive-icon>