Added adaptive icon support.

Also moved our non-adaptive app icon into the mipmap folders, which is
where they should be.

Fixes #7607

AndroidManifest.xml

@@ -93,8 +93,7 @@
     <uses-permission android:name="org.thoughtcrime.securesms.permission.C2D_MESSAGE" />
 
     <application android:name=".ApplicationContext"
-                 android:icon="@drawable/icon"
-                 android:roundIcon="@drawable/icon_circle"
+                 android:icon="@mipmap/ic_launcher"
                  android:label="@string/app_name"
                  android:supportsRtl="true"
                  tools:replace="android:allowBackup"

res/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+    <background android:drawable="@color/signal_primary"/>
+    <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+</adaptive-icon>