Fix validation of system paths in installd.

System apps are now installed under their own directory
(system_app_dir/app_dir/app.apk). The new path doesn't pass installd
validation because of obsolete checks which verify that the path does
not contain subdirectories past the system_app_dir.

The CL fixes the validation to accept at most on subdirectory.

Bug: 17109858

(cherry picked from commit c597b6dd)

Change-Id: Ic5f15d1864c6af9f4c4b07dc27244ebbb521ad5e