adb: fix `push --sync` with multiple inputs.

Previously, when push --sync was used with multiple files, we tried to
stat a remote file with pending acknowledgements still in the socket
buffer, which leads to an abort when we read ID_OKAY instead of the
response we were expecting.

This patch changes the behavior to always wait for acknowledgements if
we're pushing with --sync (or `adb sync`). This results in a ~100ms
regression during a 10 second sync of every file on the system image,
but that's not very much, so perhaps we should consider doing this
always.

Bug: http://b/166155032
Test: test_device.py
Change-Id: I7110243071ba5fbaa77dcc76f42a19d9ed2b4195