Avoid zombies on androidscreencast

When using a third party screen capture program called androidscreencast
(http://code.google.com/p/androidscreencast/) to get the framebuffer of
the device, there are tons of screencap zombie processes got left behind.
The issue is also mentioned here: http://code.google.com/p/android/issues/detail?id=22836

.
The cause of the issue is that adbd spawns off screencap processes,
and these child processes were not waited to be finished.
This change fixes the issue.

Change-Id: Ife928d65ecf6a2ff39b8b72ddba930fda6733a00
Signed-off-by: Yuriy Zabroda <yuriy.zabroda@ti.com>