Replace pkg_path with GetPkgPath() method in InstallerContext

Replace pkg_path with GetPkgPath() method in InstallerContext

pkg_path is always equal to root_application_path + pkgid.
Storing this information in separate variable is problematic,
because it requires to remember to update pkg_path after
root_application_path or pkgid is changed.

Submit with:
- https://review.tizen.org/gerrit/#/c/154196/
- https://review.tizen.org/gerrit/#/c/154904/

Change-Id: I403ef165c458091bc71ca9c80175ea7666ba2254
Signed-off-by: Damian Pietruchowski <d.pietruchow@samsung.com>