App 2.6.0: floating button to open the app

The MG4's launcher does not list third-party apps, so the app could
only be opened by reinstalling it. Now a small round MG-red button
floats over the car's screens: tap opens the app, drag moves it (the
position is kept and clamped to the screen), and it hides while the
app itself is open. A switch in the Upload tab turns it off.

The always-on service owns the overlay, so the button is there from
boot. It needs the overlay permission (SYSTEM_ALERT_WINDOW), which the
car build has through the system UID; without it, or if the window
manager refuses, the button stays away and nothing crashes, and the
Upload tab offers the system permission screen.

Tested on an Android 9 AOSP emulator at the head unit's 1920x1080 /
160 dpi with the car APK (installs as UID 1000): button appears over
the home screen, tap opens the app, hidden while open, drag moves it
without opening the app, position survives a force-stop, the switch
works through the UI. FloatingButtonGeometry has 3 unit tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/src/test/java/com/evsuite/abrp/FloatingButtonGeometryTest.java

@@ -0,0 +1,37 @@
+package com.evsuite.abrp;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+/** The floating button's position and tap rules — pure JVM. */
+public class FloatingButtonGeometryTest {
+
+    @Test
+    public void startsAtTheRightEdgeAboutFortyPercentDown() {
+        // 1920 x 1080 head unit, 64 px button, 16 px margin.
+        assertArrayEquals(new int[]{1840, 432}, FloatingButtonGeometry.defaultPosition(1920, 1080, 64, 16));
+    }
+
+    @Test
+    public void staysFullyOnScreen() {
+        assertEquals(0, FloatingButtonGeometry.clamp(-50, 64, 1920));
+        assertEquals(1856, FloatingButtonGeometry.clamp(5000, 64, 1920));
+        assertEquals(700, FloatingButtonGeometry.clamp(700, 64, 1920));
+        // A saved position from a larger screen still lands inside a smaller one.
+        assertEquals(720 - 64, FloatingButtonGeometry.clamp(1000, 64, 720));
+        // A screen smaller than the button pins it to the corner instead of going negative.
+        assertEquals(0, FloatingButtonGeometry.clamp(10, 64, 40));
+    }
+
+    @Test
+    public void aTouchWithinTheSlopIsATapAnythingFurtherADrag() {
+        assertTrue(FloatingButtonGeometry.isTap(0, 0, 8));
+        assertTrue(FloatingButtonGeometry.isTap(5, -6, 8));    // 7.8 px
+        assertFalse(FloatingButtonGeometry.isTap(6, 6, 8));    // 8.5 px
+        assertFalse(FloatingButtonGeometry.isTap(0, 40, 8));
+    }
+}