Set proguard flags

Without it, email won't work properly and many of tests will fail.

Bug 2482363

Android.mk

@@ -28,6 +28,8 @@ LOCAL_JAVA_STATIC_LIBRARIES := android-common
 
 LOCAL_PACKAGE_NAME := Email
 
+LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+
 include $(BUILD_PACKAGE)
 
 # additionally, build unit tests in a separate .apk

proguard.flags

@@ -153,3 +153,7 @@
 
 -keep class com.android.email.mail.Flag
 
+-keepclasseswithmembers class com.android.email.mail.PackedString$Builder {
+  <init>(...);
+  *** get(...);
+}