Enable shrinking for EasterEgg

This reduces the dex size by ~1.5MB (~85%). Note that we intentionally
use a very conservative Proguard config to minimize risk, as this target
isn't as widely used or tested as other system apps. In any case, most
of the reduction comes from trimming transitive deps, as the app layer
is relatively thin.

Test: m + toggle several easter eggs
Bug: 203241149
Change-Id: I5e13e2246053fde2c4ac71e38a28f8de5c6c51c7

packages/EasterEgg/Android.bp

@@ -36,7 +36,7 @@ android_app {
     certificate: "platform",
 
     optimize: {
-        enabled: false,
+        proguard_flags_files: ["proguard.flags"],
     },
 
 	static_libs: [

packages/EasterEgg/proguard.flags

@@ -0,0 +1,4 @@
+# Note: This is a very conservative keep rule, but as the amount of app
+# code is small, this minimizes any maintenance risks while providing
+# most of the shrinking benefits for referenced libraries.
+-keep class com.android.egg.** { *; }