adb: fix authentication when reconnecting.

If we have multiple keys available for authentication (ADB_VENDOR_KEYS
+ the one in ~/.android), we will still have keys in our list of
avilable keys after we've successfully connected. A subsequent
reconnection will start authorizing using the list of keys after the
key that actually worked, resulting in that session being unauthorized
until another reconnection happens. Clear the key list before
reconnecting to fix this. (We could do this after successfully
connecting, but we need to do this before reconnecting anyway, because
our connection could have died during authorization.)

Bug: http://b/117267347
Test: `adb connect foo; adb -s foo reconnect device` with ADB_VENDOR_KEYS
Change-Id: Ieb7dcc28e333c89ae0d75f97e89bcd1b571cb299