SConscript android cleanups

SConscript android cleanups

Rewrite if cmp()...elif cmp() sequence to not use slicing
  no more "magic numbers" referring to substring lengths
Update some comments
Update resource/csdk/connectivity/build/android/SConscript to
  be closer to build_common/android one
Pass error message to Exit instead of doing Exit(1) - this
  sends the error msg to stderr instead of stdout (convention)

Bug: https://jira.iotivity.org/browse/IOT-1745
Change-Id: I094cd6ecc6fecd1055f8742dd33f45d4134c2876
Signed-off-by: Mats Wichmann <mats@linux.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21153
Tested-by: Phil Coval <philippe.coval@osg.samsung.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>