adb: Mask SIGTTIN so that I/O works when backgrounded.

SIGTTIN gets sent to the process when we attempt to read from
stdin when we're in the background, which stops the process by
default. Mask the signal so that the read returns -1 with errno
set to EIO, instead.

Change-Id: I4ae626b0670c05a1a05165539b9eed709e83d536