Expose FileStoreAttributes.setBackground()

Expose FileStoreAttributes.setBackground()

The FS.setAsyncFileStoreAttributes() static method calls
FileStoreAttributes.setBackground() as its implementation, but there are
other public attributes on this inner class already and there isn't a
real reason why this needs to be private.

By making it public we allow callers to be able to invoke it directly.
Although it doesn't appear that it would make a difference, by calling a
static method on the FS class, all static fields and the transitive
closure of class dependencies must be loaded and initialised, which can
be non-trivial.

Callers referring to FS.setAsyncFileStoreAttributes() may be replaced
with FS.FileStoreAttributes.setBackground() with no change of behaviour
other than improved performance due to less class loading required.

Bug: 560527
Change-Id: I9538acc90da8d18f53fd60d74eb54496857f93a5
Signed-off-by: Alex Blewitt <alex.blewitt@gmail.com>