Move (almost all) remaining core library instrumentation tests to Robolectric.

There are 4 tests which can't currently be moved:
 - DownloadManagerTest explicitly uses the main looper which isn't easily
   supported because the test runs on this thread.
 - ContentDataSourceTest uses an AssetFileDescriptor which wraps a
   ParcelFileDescriptor. It seems Robolectric doesn't correctly forward the
   inner wrapped file descriptor leading to NPE.
 - CacheContentIndexTest and SimpleCacheSpanTest both work fine with Gradle
   but fail with seemingly valid test failures on Blaze.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185366678

extensions/flac/src/androidTest/java/com/google/android/exoplayer2/ext/flac/FlacExtractorTest.java

@@ -34,11 +34,14 @@ public class FlacExtractorTest extends InstrumentationTestCase {
   }
   }
 
 
   public void testSample() throws Exception {
   public void testSample() throws Exception {
-    ExtractorAsserts.assertBehavior(new ExtractorFactory() {
+    ExtractorAsserts.assertBehavior(
+        new ExtractorFactory() {
           @Override
           @Override
           public Extractor create() {
           public Extractor create() {
             return new FlacExtractor();
             return new FlacExtractor();
           }
           }
-    }, "bear.flac", getInstrumentation());
+        },
+        "bear.flac",
+        getInstrumentation().getContext());
   }
   }
 }
 }

library/core/src/test/AndroidManifest.xml

+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2016 The Android Open Source Project
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    package="com.google.android.exoplayer2.core.test">
+
+  <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="26"/>
+
+</manifest>