Allow #disconnect to be called safely on connection timeout (2/n)

Now #disconnect won't throw if the previous #connect failed due to
timeout. Note we also introduce generation id so we won't receive
notifications or modifications from the previous client to disrupt
UiAutomation's status when making the next connection.

Bug: 147785023
Test: m
Change-Id: Idf77207124494bd78770b8ea5d9ac4b1fd1a490a