Add proguard rule to keep the class for all implementations of FlutterPlugin (flutter#154677)

Fixes flutter#154580, for the time being. 

We should follow up to determine why this is necessary, but I think it is important to un-break these specific plugins for now.

Author: gmackall

packages/flutter_tools/gradle/flutter_proguard_rules.pro

@@ -5,3 +5,10 @@
 
 # The android.** package is provided by the OS at runtime.
 -dontwarn android.**
+
+# In some cases, R8 is incorrectly stripping plugin classes. Keep
+# all implementations of FlutterPlugin until we can determine
+# why this is the case.
+# See https://github.com/flutter/flutter/issues/154580.
+-if class * implements io.flutter.embedding.engine.plugins.FlutterPlugin
+-keep,allowoptimization,allowshrinking,allowobfuscation class <1>