vold: add ro.vold.umsdirtyratio property

It allows control of what /proc/sys/vm/dirty_ratio 
is used when USB mounting.

At some platforms like the HTC Desire (bravo)
and Samsung Galaxy S family (i9000x) we get a _HUGE_
performance boost when writing to the sdcard when USB
mounting if this is set to 20 (which is the default)
on the devices above.

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I86b7bbd02695fb0c19f39448714a219257bf806f
Merge commit 'refs/changes/73/4873/6' of https://gerrit.omnirom.org/android_system_vold into HEAD