OkHttp extension - Improved configuration

- Upgrade to latest version
- Use api dependency, since application code that uses the
  extension more has to use OkHttp directly to make an
  OkHttpClient instance
- Add proguard configuration

Issue #4059

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191422594

extensions/okhttp/build.gradle

@@ -21,6 +21,7 @@ android {
     defaultConfig {
         minSdkVersion project.ext.minSdkVersion
         targetSdkVersion project.ext.targetSdkVersion
+        consumerProguardFiles 'proguard-rules.txt'
     }
 
     lintOptions {
@@ -32,7 +33,7 @@ android {
 dependencies {
     implementation project(modulePrefix + 'library-core')
     implementation 'com.android.support:support-annotations:' + supportLibraryVersion
-    implementation('com.squareup.okhttp3:okhttp:3.9.0') {
+    api('com.squareup.okhttp3:okhttp:3.10.0') {
         exclude group: 'org.json'
     }
 }

extensions/okhttp/proguard-rules.txt

@@ -0,0 +1,8 @@
+# Proguard rules specific to the OkHttp extension.
+
+# Options specified by https://github.com/square/okhttp/blob/master/README.md
+-dontwarn okhttp3.**
+-dontwarn okio.**
+-dontwarn javax.annotation.**
+-dontwarn org.conscrypt.**
+-keepnames class okhttp3.internal.publicsuffix.PublicSuffixDatabase