Add public SynthesisRequestV2 constructor.

While implementing the new TTS API, I found out that it's very difficult
to create an SynthesisRequestV2 instance for testing purposes - it's
a final class, with no publicly visible constructors.

Bug: 8259486
Change-Id: I88b84fd8ad1ac6960f3932863ca758657f9547ff
2 files changed