Explicitly label vold dependecies in /proc

1. Labeled:
/proc/cmdline -> proc_cmdline

2. Removed access to proc label from vold domain.

3. Added access proc_cmdline to these domains:
init, kernel, vold

4. Also, added proc_drop_caches access to vold.

Bug: 66497047
Test: device boots without selinux denials to new labels
Change-Id: Ic88d11b7e56b07c0e8bd874e7f72788922a218e3