[DROID] Ease mount point blacklist condition

Android doesn't require USB devices be mounted under /mnt/usb.  In fact
there SEEMS to be no requirements or useful metrics for determining info
about mounted devices besides the "default external storage device"
which is always an SD card.  However, they do all SEEM to be handled by
Vold, the volume manager daemon and given a device node under
/dev/block/vold/.  It also SEEMS, from the four vendors I have on hand,
that SD card mount points alwasy contain the string "sdcard".  Using
these two rules get's me auto sources for USB devices on AML devices.
[DROID] Read /proc/mounts in one go to avoid race.

/proc/mounts is only guaranteed atomic for the current read operation.
[DROID] Only add one mount point per device

I've observed some prettys strange behavior on AML devices regarding
reading /proc/mounts.  Somehow there are multiple, unique mount points
for some devices when reading /proc/mounts with fopen/fgets.  However
they don't exist if "cat /proc/mounts" is issued from the shell.
Baffling...
Merge pull request #4236 from t-nelson/droid_removable_storage_fixes

Droid removable storage fixes