Fix resolveAndroidApiVersion when running under Robolectric (ReactiveX#4912)

PlatformDependent.resolveAndroidApiVersion() tries to determine the
Android SDK version by reading the android.os.Build$VERSION#SDK_INT
field. When running under Robolectric, the class will be found (since
Robolectric bundles an original android.jar). However, the method is
using the system class loader for loading it (instead of the Robolectric
instrumenting class loader), which will not instrument the class to run
in the JVM. As a result, static initialization of the class fails with
an UnsatisfiedLinkError when calling SystemProperties.get(), which calls
native method native_get.

I fixed by using the default class loader instead. This would only make
a difference in the extremely rare case that the Android application
installs a class loader that's not capable of finding Android classes.
I tested that this behaves as expected on a device, fixes the issue
with Robolectric, and detects it's not Android otherwise (as proven
by the unit test).

Author: cesar1000

src/main/java/rx/internal/util/PlatformDependent.java

@@ -15,9 +15,6 @@
  */
 package rx.internal.util;
 
-import java.security.AccessController;
-import java.security.PrivilegedAction;
-
 /**
  * Allow platform dependent logic such as checks for Android.
  *
@@ -67,7 +64,7 @@ public static int getAndroidApiVersion() {
     private static int resolveAndroidApiVersion() {
         try {
             return (Integer) Class
-                    .forName("android.os.Build$VERSION", true, getSystemClassLoader())
+                    .forName("android.os.Build$VERSION")
                     .getField("SDK_INT")
                     .get(null);
         } catch (Exception e) { // NOPMD
@@ -76,20 +73,4 @@ private static int resolveAndroidApiVersion() {
             return ANDROID_API_VERSION_IS_NOT_ANDROID;
         }
     }
-
-    /**
-     * Return the system {@link ClassLoader}.
-     */
-    static ClassLoader getSystemClassLoader() {
-        if (System.getSecurityManager() == null) {
-            return ClassLoader.getSystemClassLoader();
-        } else {
-            return AccessController.doPrivileged(new PrivilegedAction<ClassLoader>() {
-                @Override
-                public ClassLoader run() {
-                    return ClassLoader.getSystemClassLoader();
-                }
-            });
-        }
-    }
 }

src/test/java/rx/internal/util/PlatformDependentTest.java

@@ -19,9 +19,18 @@
 
 import rx.TestUtil;
 
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+
 public class PlatformDependentTest {
     @Test
     public void constructorShouldBePrivate() {
         TestUtil.checkUtilityClass(PlatformDependent.class);
     }
+
+    @Test
+    public void platformShouldNotBeAndroid() {
+        assertFalse(PlatformDependent.isAndroid());
+        assertEquals(0, PlatformDependent.getAndroidApiVersion());
+    }
 }