Postpone loading app context until instrumentation is set up

The application context needs instrumentation data to be in place
prior to construction so that the class loader is initialized using
the correct paths.

Removes unnecessary clearClassLoader() workaround that was previously
added to fix instrumentation tests.

Bug: 22627299
Change-Id: Ibf8e69e037189a9a563f0b68cfe333461726b71d