adb: setsid() for adb host server.

To create a daemon for adb host server, we should call setsid()
for the daemon process. However, previously we call setsid() for
the adb client process, which results in nothing but EPERM error.

Bug: 26982628
Change-Id: I2763ae3d5a243706927d7ef6af5095138c0ce2d8
(cherry picked from commit 6bf323b9)