Set default visibility to private

Added visibility to apache-xml-testdex even though it is only used in
.mk files to record the dependency as the name is constructed
dynamically by adding "-testdex" suffix to a list of other dependencies
and so cannot be found by searching.

Bug: 134379140
Test: m droid
Change-Id: I64d4cdb2fbed80a0d39f06ae2656367ce8c69fbc
1 file changed