Disallow adding nested conditional class loader context.

Nested conditional context doesn't make sense because conditional
context is only needed for compatibility libraries, and those are only
used by apps, and apps are always at the root of class loader context
tree. Therefore, trying to add conditional nested context can only mean
an error somewhere on the way.

Test: lunch aosp_cf_x86_phone-userdebug && m
Bug: 132357300
Change-Id: Iadeaaf89fdb11cb23b107c6fb074b1bc765256be