buildscripts: Build android instrumentation tests in android CI

Binder's :build was missing. Cronet build failed without specifying
Java 8 because of the transitive Guava dependency.

buildscripts/kokoro/android.sh

@@ -31,6 +31,8 @@ buildscripts/make_dependencies.sh
     :grpc-android-interop-testing:build \
     :grpc-android:build \
     :grpc-cronet:build \
+    :grpc-binder:build \
+    assembleAndroidTest \
     publishToMavenLocal
 
 if [[ ! -z $(git status --porcelain) ]]; then

cronet/build.gradle

@@ -28,6 +28,10 @@ android {
             consumerProguardFiles 'proguard-rules.pro'
         }
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     testOptions { unitTests { includeAndroidResources = true } }
     lintOptions { disable 'InvalidPackage' }
 }