Improve proguard rules for androidx.startup.

* Keep the no-argument constructor but allow other methods to get
  optimized.

Test: N/A
Change-Id: I7d609bda6e6c16b9ee32b9ea9c5a09ce15605064
diff --git a/startup/startup-runtime/proguard-rules.pro b/startup/startup-runtime/proguard-rules.pro
index 78991b3..0826b37 100644
--- a/startup/startup-runtime/proguard-rules.pro
+++ b/startup/startup-runtime/proguard-rules.pro
@@ -1,3 +1,6 @@
 # This Proguard rule ensures that ComponentInitializers are are neither shrunk nor obfuscated.
 # This is because they are discovered and instantiated during application initialization.
--keep class * extends androidx.startup.ComponentInitializer
+-keep class * extends androidx.startup.ComponentInitializer {
+    # Keep the public no-argument constructor while allowing other methods to be optimized.
+    public <init>();
+}