Remove AAudioTesting.h from headers to be exported to NDK

but include the associated names in the global symbol export list, so
that apps that receive AAudioTesting.h outside of NDK can access the
symbols without dlsym().

Test: builds OK
Change-Id: If68f35145d68f359ebb7995ffcf9c9b8f9be6eb1
(cherry picked from commit 4257fd9a)