[speech] unbind from recognition service

Currently, we only remove the service reference, but rely on the timeout
built into `ServiceConnector` to actually unbind. This is causing a
service leak.

We also move `SpeechRecognizer#destroy()` to be handled on the Handler
(just like all other calls) in order to fix racing.

Bug: 313604307
Bug: 311124261
Test: cts
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:3e14c03272a0e1bfb3b50839efcaae88461b730d)
Merged-In: I5384916da98b286f869a32857079087105ca32f9
Change-Id: I5384916da98b286f869a32857079087105ca32f9