vold: support more than 1 UMS share

Used for devices with both mountable internal and external sdcards.

This is a forward-port and clean up of the 10.2 code (which contains
a lot of deadcode.)

Original authors:

    Alex Zepeda <alex@inferiorhumanorgans.com>
    Genokolar <genokolar@gmail.com>
    Josh Stone <cuviper@gmail.com>
    mik_os <kiril.mik.os@gmail.com>
    Oran Avraham <oranav@gmail.com>
    Pawit Pornkitprasan <p.pawit@gmail.com>
    Tony Layher <layhertony@gmail.com>

Change-Id: I13b5c0b58e3d420bcdf894b67f2ef7eab88eb7f0
vold: add ro.vold.umsdirtyratio property

This allows control of what /proc/sys/vm/dirty_ratio is used when USB mounting.
Some platforms like the HTC Desire (bravo) 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 HTC Desire.

This addresses this issue: http://code.google.com/p/cyanogenmod/issues/detail?id=2949

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

Change-Id: Icc518132d939d84ca101c83eea26f88e089034da
Merge "vold: support more than 1 UMS share" into android-4.4
Merge remote-tracking branch 'upstream/android-4.4' into android-4.4

Conflicts:
	Android.mk