Replaces Toast with a custom minimal snackbar-like TextView.

- Snackbar requires the material library, which (even after
  automatic optimization) increases the APK size from ~300kb
  to ~2.5MB.
- Toasts can normally be displayed without any permissions from
  foreground apps. However, despite rendering UI
  AccessibilityServices run in the background so they cannot
  post Toasts without requesting POST_NOTIFICATION permission
  from the user.

This minimal TextView appears in the same style as a Snackbar but
without any extra dependencies.

Bug: 261252772
Test: Call showSnackbar(), observe custom snackbar UI
Change-Id: Ide2cf4640ff930afdb2ec3ff4eda833058a0b9f6