Add the latest version of Gmail

The following changes was made to the Proguard configuration for R8
processing to succeed:

1. Added the line -dontwarn sun.net.spi.nameservice.NameServiceDescriptor
2. Removed Signature from -keepattributes
3. Uncommented -verbose
4. Uncommented all -checkdiscard rules

Change-Id: Ie28fe241b7d378b74254cf22b705d0e264d9ed8c
diff --git a/build.gradle b/build.gradle
index 40a9ef2..c81d94d 100644
--- a/build.gradle
+++ b/build.gradle
@@ -145,6 +145,7 @@
 
 def x20Dependencies = [
     "third_party": [
+        "gmail/gmail_android_170604.16.tar.gz",
         "gmscore/v4.tar.gz",
         "gmscore/v5.tar.gz",
         "gmscore/v6.tar.gz",