[Core, Android] Fix to address the elusive 'Sharing violation on path PropertyStore.forms.tmp' (#1075)

* use a semaphor for actual saving piece
make reading serialized propertystore readonly to help with collisions.

* change over to async Task and await OnStateChanged() method to try and mitigate what might be the race condition causing the ' Sharing violation on path PropertyStore.forms.tmp' error

* update Semaphore name

* remove private specifier per the style guide

* Update docs and add warning suppression for broken mono warning

* ... and adding the warning suppression to Release mode, as well.

* -add try...finally block so we can guarantee that the semaphore is released if there's an exception while saving the properties.
-remove the legacy code that the semaphore replaces

* Update docs

* Attempting to fix docs

* Trying again to get docs to pass

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