Make Keyboard object immutable except shift state

This is the first step to implement suggestions pane as mini keyboard.

Bug: 5023981
Change-Id: I90ffbde0fda19b4be68add449310997b56bf6904
diff --git a/java/proguard.flags b/java/proguard.flags
index 7ce6f41..44416ec 100644
--- a/java/proguard.flags
+++ b/java/proguard.flags
@@ -30,3 +30,7 @@
 -keep class com.android.inputmethod.latin.SettingsActivity {
   *;
 }
+
+-keep class com.android.inputmethod.keyboard.internal.MiniKeyboardBuilder$MiniKeyboardLayoutParams {
+  <init>(...);
+}