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
Change-Id: I13abb52c0016610ff436f6a26bb6b3b85dc4dfb0