binderRpcTest: conditional experimental protocol

Previously this test failed in REL mode and required a
change. This was done so that we made sure to freeze it for
release. Now that we have TARGET_RELEASE, we need to make this
conditional.

Bug: 305786304
Test: binderRpcTest on '-next' configuration
Change-Id: I726c71399c7e469698abc630a006ce089dc4bca8