adbd: listen to all functionfs events.

Monitor for FUNCTIONFS_UNBIND as well, so that in the case where we get
FUNCTIONFS_BIND, FUNCTIONFS_UNBIND, FUNCTIONFS_BIND, we don't trigger an
assertion failure from seeing two FUNCTIONFS_BINDs.

Bug: http://b/129134256
Test: manual
Change-Id: I80af5f4b833513e932262638b9f8d76bbcb35504