icon: add regular png icon for compatibility with some launchers

See https://github.com/termux/termux-app/issues/991.

Adaptive icon will continue to be automatically used if supported.

app/src/main/res/drawable/ic_launcher.xml

@@ -1,34 +0,0 @@
-<vector xmlns:android="http://schemas.android.com/apk/res/android"
-    android:height="108dp"
-    android:width="108dp"
-    android:viewportWidth="108"
-    android:viewportHeight="108">
-
-    <path
-        android:fillColor="#000000"
-        android:pathData="M18,54
-                          A36,36 0 1,1 90,54
-                          A36,36 0 1,1 18,54 Z" />
-
-    <!-- Keep in sync with adaptive ic_foreground.xml: -->
-    <path
-        android:fillColor="#FFFFFF"
-        android:pathData="M34,38
-                          h6
-                          l12,16
-                          l-12,16
-                          h-6
-                          l12,-16
-                          "
-        />
-
-    <path
-        android:fillColor="#FFFFFF"
-        android:pathData="M56,66
-                          h18
-                          v4
-                          h-18
-                          "
-        />
-
-</vector>