Add new API for getting filtered count

Add new API for getting filtered count

usage)
var filter = new PackageFilter();
filter.Filters.Add(PackasgeFilter.Keys.Removalble, true);
var pkgList1 = PackageManager.GetPackage(filter);
var pkgList2 = PackageManager.GetPackage(filter);
int count = filter.GetCount(); // get count from latest filtered result.
int count = filter.GetCount(pkgList1); // get count from the given filtered result.

Change-Id: Idd111de78964bd1bc81e4703945b4cc8d1fb8987
Signed-off-by: jongmyeongko <jongmyeong.ko@samsung.com>