SoundTriggerHelper: improve in call behavior

Fix 3 issues in SoundTriggerHelper when a call is active:
1) Do not actually load a sound model when recognition start is requested
but disabled because of and active call or any other reason. Previously
we were loading the sound model even if actual start was deferred until
enabled.
2) Allow to start recognition while disabled: the recognition start
request is stored as pending and will be executed when enabled again.
3) Process call state change with enough delay when call ends to allow
time for the HAL to process the call disconnect notification before re
enabling pending recognitions.

Also remove assumption on valid model handle values.

Test: repro steps in b/152723355
Bug: 152723355
Change-Id: Id46ae8c285426ed2ace8a6d80478fecb98f4ec33