adb: fix adbd feature parsing for no features.

Previously the transport features list was only overwritten if a
new feature list was found. However, adbd can reuse the same atransport
object even if the adb server is killed and restarted, so the feature
list was not cleared properly if the newly started adb server didn't
provide one.

This CL fixes the bug by clearing the transport features list whenever
a connection banner is parsed.

Bug: http://b/24405971
Change-Id: Ia6ee6c9a46a621534681f6d4d7df77156b885eb9