Remove mkdir() side effect, add .nomedia, utils.

Before this change, FUSE lookup() would have the side effect of
creating the directory on behalf of apps.  This resulted in most
directories being created just by Settings trying to measure disk
space.  Instead, we're switching to have vold do directory creation
when an app doesn't have enough permissions.

Create fs_mkdirs() utility to create all parent directories in a
path as needed.  Allow traversal (+x) into /storage directories.

Fix FUSE derived permissions to be case insensitive.  Mark well-known
directories as .nomedia when created.

Bug: 10577808, 10330221
Change-Id: I53114f2e63ffbe6de4ba6a72d94a232523231cad