Reverting back to Android-21 as the move to Android-23 causes Android

Reverting back to Android-21 as the move to Android-23 causes Android
to crash on M devices. The current build scripts will need to be updated
to generate shared libs with soname.
Till the scripts are updated, Android-21 will be used as it works even
on M devices.

Change-Id: I33e0ccc19fb406b3508d888f7b04617c7d6e0868
Signed-off-by: Rahul Rahul <rahul.rahul@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/4873
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>