Allow EuiccService IPCs to happen in parallel.

The (currently) undocumented behavior of oneway AIDLs is to serialize
IPCs made from the same client IBinder object. This is undesirable in
EuiccService as we want to respond to short-lived calls (like listing
subscriptions or getting the EID) while long-lived calls (like
download) are in progress. Introduce our own thread pool to work
around this behavior.

This also fixes a bug where uncaught exceptions thrown in the
implementation of an EuiccService method would be logged but never
trigger a callback on the client. Such exceptions now correctly crash
the app and trigger error callbacks in any active callers.

Bug: 62535655
Test: TreeHugger + verified subscription list during a download
Change-Id: I8b75af7fdc72117a163d81f7bf1447f6cc12ec6d