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/android_app_conversion_test.go

@@ -74,7 +74,8 @@ android_app {
         package_name: "com.google",
         resource_dirs: ["resa", "resb"],
         manifest: "manifest/AndroidManifest.xml",
-        static_libs: ["static_lib_dep"]
+        static_libs: ["static_lib_dep"],
+        java_version: "7",
 }
 `,
 		expectedBazelTargets: []string{
@@ -87,6 +88,7 @@ android_app {
     ]`,
 				"custom_package": `"com.google"`,
 				"deps":           `[":static_lib_dep"]`,
+				"javacopts":      `["-source 1.7 -target 1.7"]`,
 			}),
 		}})
 }