base

Fix TTS instantiation from webview accessibility handlers.

The TTS instantiated from here shouldn't clobber any
existing TTS objects opened within the same package context.

Ideally, the TTS API should work fine with multiple TTS object
instances within the same package context but making that happen
correctly is a larger change.

bug:5659758
Change-Id: Ia1f63c61b9f12ac92ff42a427a004d414e42a759