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.

It makes it possible to undo or control the effect of this commit:
https://github.com/CyanogenMod/android_system_vold/commit/a28056b38275003895ff5d9576681aca01544822

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I86b7bbd02695fb0c19f39448714a219257bf806f