bp2build converts java_version property to javacopts attribute

This CL also converts `external/rappor` (which already set `java_version` to `1.7`) to be bazelable to testify the changes.

Results from `b build //external/rappor && cat bazel-bin/external/rappor/librappor.jar-0.params`: https://paste.googleplex.com/5518725462622208.

Test: go test ./bp2build/...
Bug: 227618664
Change-Id: I8d370d4639f70fba51e6de6ceb7bcb5ace9ccd91

bp2build/java_plugin_conversion_test.go

@@ -39,6 +39,7 @@ func TestJavaPlugin(t *testing.T) {
     libs: ["java-lib-1"],
     static_libs: ["java-lib-2"],
     bazel_module: { bp2build_available: true },
+    java_version: "7",
 }
 
 java_library {
@@ -66,6 +67,7 @@ java_library {
         "a.java",
         "b.java",
     ]`,
+				"javacopts": `["-source 1.7 -target 1.7"]`,
 			}),
 		},
 	})