speech: Update alternative spans API

As per API council’s advice, builders have been removed from both
AlternativeSpan and AlternativeSpans classes. The objects are now built
via a public constructor which takes parameters for all fields.

Tests are updated so they reflect the aforementioned changes.

Bug: 236591093
Test: CTS
Change-Id: Ie6bdb6d5167407e8d77eb148c343c263119e4863