Update UI for Flash notifications

1. Hide the preview button when it's disabled
2. Change the text from “Done” to “Save” in the color selector dialog

Flag: EXEMPT simple UI change
Bug: 356978471
Test: manually, video attached on the bug
Test: atest FlashNotificationsPreviewPreferenceControllerTest
Change-Id: I2e6e697ad10f0207602d613085663cdd82521ea2