Implement Ctrl+B/I/U shorcut

These shortcut works as follows:

- If the selected region is empty, do nothing.
- If the selected region has at least one character that is not styled,
  e.g. bold, apply bold style.
- If the selected region is fully styled, e.g. bold, remove bold style
  from the region.

The default TextView/EditText supports styled editing, but it is used
commonly as plain text fields, e.g. email address in the form, etc.
So, not to style the text in such places, disable styled shortcut by
 default and let developers to enable it when necessary.

Bug: 240628246
Test: TextViewStyleShortcutTest
Change-Id: Id0f1cd85eb198553a852c5c2b628f5d03a2741e1