adb: hopefully deflake test_adb.py.

The reconnection test is spuriously failing on test infrastructure for
unclear reasons, which might be due to a race between the connection
attempt and the first command we send. Insert a sleep to hopefully
reduce the flakiness.

Bug: http://b/123247844
Test: ./test_adb.py (but it didn't fail for me in the first place)
Change-Id: Ic36924c16bae424accfec700af4623794fd1f123