binder: TLS checks trigger properly.

Previously, libbinder_tls ignores the result of
isTriggeredPolled() by always returning OK regardless
of whether the shutdown trigger is triggered or not, causing
program to continue when it shouldn't be. Return the status
properly like FdTrigger::triggerablePoll:

- If any error during poll() return the code
- If shutdown, return -ECANCELED (new in this CL for TLS)
- Otherwise return OK

Refactor RpcTransportTest so that we can add a new test
to check that trigerablePoll() returns -ECANCELED in the
above case.

Test: binderRpcTest
Fixes: 199309623

Change-Id: Ia545ba71cc10be5c46f722a5d3e699f89e1bc70c