More robust ref counting in EuiccConnector.

The current implementation has two holes:

-If the LPA crashes between the command being dispatched successfully
(a oneway IPC) and the callback being executed, the operation will
never be considered finished.

-The callbacks happen on a binder thread, whereas all other access to
the ref counter happens on the handler thread.

We address both of these by moving the callbacks to the state
machine's handler thread and by keeping track of all callbacks for all
pending commands. If the state machine is about to leave the connected
state, because the connection is now severed, we call all of these
callbacks (with a generic "service unavailable" error).

Fixes: 37480432
Test: TreeHugger + Test of a crashing LPA
Change-Id: Icc31317d43cffa28f386478f04f7dd99622eb3c9