Update TTS speech rate option with percentages

Previews on Amati and Watson:
https://screenshot.googleplex.com/3D3fcZCg4uwRV9F
https://screenshot.googleplex.com/8Nd2Ewuij6dWuoX

Test: Manually on Amati and Watson
Bug: 242684672
Change-Id: I41b9f51d443ecff8267062a3bf3e93e00f51a5ff
1 file changed