adb: defer acknowledgement of pushed files until the end.

Previously, we were waiting for the other end to respond after every
file sent, which results in massive slowdown when there's any amount of
latency on the transport.

This improves performance on a cuttlefish instance with ~7ms RTT from:

    system/: 2037 files pushed, 0 skipped. 2.8 MB/s (762803979 bytes in 262.964s)

to:

    system/: 2037 files pushed, 0 skipped. 11.9 MB/s (762803979 bytes in 61.278s)

Bug: https://issuetracker.google.com/150827486
Test: ./test_device.py
Change-Id: I3a0c893faa5d455cc6ccbc86915a17e1b5abbfbe
(cherry picked from commit 64ff82ba)