Fix Cronet extension build and test.

Recently added Java 8 features in the cronet extension and the linked native libs
require to enable Java 8 desugaring in gradle. Moreover, junit.assertThrows is not
available in our version and its usage has been replaced by the manual check.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172077967

extensions/cronet/build.gradle

@@ -27,6 +27,11 @@ android {
     sourceSets.main {
         jniLibs.srcDirs = ['jniLibs']
     }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
 }
 
 dependencies {

extensions/cronet/src/androidTest/java/com/google/android/exoplayer2/ext/cronet/CronetDataSourceTest.java

@@ -18,7 +18,6 @@ package com.google.android.exoplayer2.ext.cronet;
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static org.mockito.Matchers.any;
@@ -163,7 +162,12 @@ public final class CronetDataSourceTest {
   public void testOpeningTwiceThrows() throws HttpDataSourceException {
     mockResponseStartSuccess();
     dataSourceUnderTest.open(testDataSpec);
-    assertThrows(IllegalStateException.class, () -> dataSourceUnderTest.open(testDataSpec));
+    try {
+      dataSourceUnderTest.open(testDataSpec);
+      fail("Expected IllegalStateException.");
+    } catch (IllegalStateException e) {
+      // Expected.
+    }
   }
 
   @Test