Add proguard rules to the AAR.

Test: Ran tests with the integration test app.
Fixes: b/159595260
Change-Id: I1e7ada99defaad2f5e8e17c32b37b096d8e91539
diff --git a/startup/startup-runtime/proguard-rules.pro b/startup/startup-runtime/proguard-rules.pro
index f55c09a..b1048d8 100644
--- a/startup/startup-runtime/proguard-rules.pro
+++ b/startup/startup-runtime/proguard-rules.pro
@@ -2,7 +2,7 @@
 # This is because they are discovered and instantiated during application initialization.
 -keep class * extends androidx.startup.Initializer {
     # Keep the public no-argument constructor while allowing other methods to be optimized.
-    public <init>();
+    <init>();
 }
 
 -assumenosideeffects class androidx.startup.StartupLogger