Handle magnet links with termux-url-opener

Fixes #1339 and #1382.

app/src/test/java/com/termux/filepicker/TermuxFileReceiverActivityTest.java

@@ -0,0 +1,32 @@
+package com.termux.filepicker;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+
+import java.util.ArrayList;
+import java.util.List;
+
+@RunWith(RobolectricTestRunner.class)
+public class TermuxFileReceiverActivityTest {
+
+    @Test
+    public void testIsSharedTextAnUrl() {
+        List<String> validUrls = new ArrayList<>();
+        validUrls.add("http://example.com");
+        validUrls.add("https://example.com");
+        validUrls.add("https://example.com/path/parameter=foo");
+        validUrls.add("magnet:?xt=urn:btih:d540fc48eb12f2833163eed6421d449dd8f1ce1f&dn=Ubuntu+desktop+19.04+%2864bit%29&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A80&tr=udp%3A%2F%2Ftracker.publicbt.com%3A80&tr=udp%3A%2F%2Ftracker.ccc.de%3A80");
+        for (String url : validUrls) {
+            Assert.assertTrue(TermuxFileReceiverActivity.isSharedTextAnUrl(url));
+        }
+
+        List<String> invalidUrls = new ArrayList<>();
+        invalidUrls.add("a test with example.com");
+        for (String url : invalidUrls) {
+            Assert.assertFalse(TermuxFileReceiverActivity.isSharedTextAnUrl(url));
+        }
+    }
+
+}