Expose generate_clear as an option.

I wasn't able to get the clear() method to inline into the
constructor when optimizations are on in proguard. As a result,
every message has an extra superfluous kept method assuming the
app never uses clear() directly.

There are a couple of instances where setting this option false is
necessary in order to get code dexing successfully without hitting
the method limit, e.g. https://goto.google.com/tltzq

In this example, I tried turning on the method/inlining/unique and
method/inlining/short optimizations before resorting to adding the
generate_clear option, but the method count did not decrease. The
clear() methods were contributing over a thousand extra methods.

Change-Id: If6a9651d6a59cdf70b1040d8248779710ac73105

src/google/protobuf/compiler/javanano/javanano_generator.cc

@@ -156,6 +156,8 @@ bool JavaNanoGenerator::Generate(const FileDescriptor* file,
       params.set_generate_clone(option_value == "true");
     } else if (option_name == "generate_intdefs") {
       params.set_generate_intdefs(option_value == "true");
+    } else if (option_name == "generate_clear") {
+      params.set_generate_clear(option_value == "true");
     } else {
       *error = "Ignore unknown javanano generator option: " + option_name;
     }