Improve android building

Improve android building

build_common/SConscript:
    - ANDROID_NDK, ANDROID_HOME, ANDROID_GRADLE option variables
    setup moved to end.  A visit if needed to respective SConscripts
    will cause the default path to be computed, this path is
    then supplied to the variable setup.  The comment in the later
    section will describe more.
build_common/android/SConscript:
    - option var setup not heeded here. Just call ndk, gradle, sdk
    SConscript if path not already set.
    - API_VER is set at top, idea is to have one place to change
    to experiment with new verisons.
    - all three paths are printed from one "print", this can easily
    be disabled if not wanted.
build_common/linux/SConscript,
build_common/windows/SConscript:
    - just pick up gradle info from SConscript, no need for the
    option var setup.
extlibs/android/gradle/SConscript:
    - clean up; version is now set in a single variable
extlibs/android/ndk/SConscript:
    - clean up; version is now set in a single variable
    - adds Mac support - untested, but why not: sdk script does it
extlibs/android/sdk/SConscript:
    - clean up; version is now set in a single variable

Change-Id: I7e4355493808eddce6899fe657657e56c339ae5d
Signed-off-by: Mats Wichmann <mats@linux.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21631
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Larry Sachs <larry.j.sachs@intel.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>