drop_caches label, vold scratch space on expanded.

Define an explicit label for /proc/sys/vm/drop_caches and grant to
the various people who need it, including vold which uses it when
performing storage benchmarks.

Also let vold create new directories under it's private storage area
where the benchmarks will be carried out.  Mirror the definition of
the private storage area on expanded media.

avc: denied { write } for name="drop_caches" dev="proc" ino=20524 scontext=u:r:vold:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=0

Bug: 21172095
Change-Id: I300b1cdbd235ff60e64064d3ba6e5ea783baf23f