Do not call exit() immediately after serving sideload

Wait for socket to close before exit(), so that last "DONEDONE" message
is correctly sent.

Test: enter recovery, sideload an OTA, verify that no read error occured
      on client side. Repeat 5 times.
Bug: 155158483
Change-Id: Ibb02548d8c03fb77f8f60cb69cff84c33903adc0