Add support for and use new path= specifier in seapp_contexts.

Extend check_seapp to accept the use of the new path= specifier
in seapp_contexts and use it to ensure proper labeling of the cache
subdirectory of com.android.providers.downloads for restorecon.

After this change, restorecon /data/data/com.android.providers.downloads/cache
does not change the context, leaving it in download_file rather than
relabeling it to platform_app_data_file.

Depends on Iddaa3931cfd4ddd5b9f62cd66989e1f26553baa1.

Change-Id: Ief65b8c8dcb44ec701d53e0b58c52d6688cc2a14
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>