Gradle: Bump android plugin to 2.0.0

Also add useProgard true, since minifyEnabled now refers to the new
experimental code shrinker.

I'm not removing proguard yet as we depend on it for the samsung
workaround. I also do not know how to port the rest of the config
options to the new shrinker.

app/build.gradle

@@ -134,6 +134,7 @@ android {
         // release builds before.
         all {
             minifyEnabled true
+            useProguard true
             shrinkResources true
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
             testProguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro', 'src/androidTest/proguard-rules.pro'

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:1.5.0'
+        classpath 'com.android.tools.build:gradle:2.0.0'
         classpath files('libs/gradle-witness.jar')
     }
 }