Refines volume key shortcut confirm dialog strings

- Provide more clear content for single and multiple services are
enabled.
- Avoid non-a11y users accidentally turning shortcut on without
reading this carefully. Hence we put "don't turn on" as the
primary action.

Bug: 155249323
Bug: 138582063
Test: atest AccessibilityShortcutControllerTest
Change-Id: I1c391bb0516bcebfbf6161b94cc0c0b80e0bb72f