AIDEGen: Add path attribute to okhttp module.

Bug: 157115477
Test: 1. aidegen Settings -n
         open merged_module_info.json
	 "okhttp": {
	     ...
	     "path": ["external/okhttp"]
         },
	 modules: "mimemap-testing-res.jar", "mimemap-res.jar"
                  "services.core.priorityboosted", "framework-tethering"
                  "bouncycastle_ike_digests",
                  "robolectric-sqlite4java-native"
                  have the 'path' attribute.
Change-Id: If3628887866c29dcc948dd427727f175a5199edc