os: add isAndroid() function

This will be used to test if the app is running on Android.

src/utils/os.test.ts

@@ -1,12 +1,23 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2021 The Pybricks Authors
 
-import { isMacOS, isWindows, prefersDarkMode } from './os';
+import { isAndroid, isMacOS, isWindows, prefersDarkMode } from './os';
 
 afterEach(() => {
     jest.resetAllMocks();
 });
 
+describe('isAndroid', () => {
+    test('is true', () => {
+        jest.spyOn(navigator, 'userAgent', 'get').mockReturnValue('Android');
+        expect(isAndroid()).toBeTruthy();
+    });
+    test('is false', () => {
+        jest.spyOn(navigator, 'userAgent', 'get').mockReturnValue('Linux');
+        expect(isAndroid()).toBeFalsy();
+    });
+});
+
 describe('isMacOS', () => {
     test('is true', () => {
         jest.spyOn(navigator, 'platform', 'get').mockReturnValue('MacIntel');

src/utils/os.ts

@@ -3,6 +3,17 @@
 
 // Utility functions for dealing with operating systems.
 
+// TODO: replace with navigator.userAgentData when it is more widely available
+// https://developer.mozilla.org/en-US/docs/Web/API/User-Agent_Client_Hints_API
+
+/**
+ * Tests if we are running on Android.
+ * @returns `true` if running on Android, otherwise `false`.
+ */
+export function isAndroid(): boolean {
+    return /android/i.test(navigator.userAgent);
+}
+
 /**
  * Tests if we are running on macOS.
  * @returns `true` if running on macOS, otherwise `false`.