IVGCVSW-3595 Implement the LoadDynamicBackends function in the Runtime class

IVGCVSW-3595 Implement the LoadDynamicBackends function in the Runtime class

 * Changed the way the handle is acquired, loaded symbols are now kept local
 * Updated the makefiles to add more test files for the dynamic backends
 * Fixed the GetSharedObjects method so that the files are parsed in
   alphabetical order
 * Updated the unit tests to make them more strict wrt the order of the
   files
 * Created a new CreateDynamicBackends method in the utils class
 * Added new unit tests for the new function
 * Added LoadDynamicBackends in the Runtime class

!android-nn-driver:1707

Change-Id: I1ef9ff3d5455ca6a7fd51cb7cfb3819686234f70
Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>