Initiate connection teardown only once

Otherwise, this gets called multiple times from different places via MSG_TEARDOWN. This causes the reconnect delay to increase with each call to scheduleReconnect(), increasing the time we stay disconnected.

This commit introduces a boolean flag preventing handleTeardown() to run twice or more until connect() was called again.

Change-Id: I3d7cb08d696be48532a61819fbb279a908919a3d