Bug 1063461 - de-xfail keyboard_manager.js and annotate private members

- Remove keyboard_manager.js from js linter xfail list (and make it jshint-compliant)
- Annotate private members of keyboard_manager.js with underscore-prefix
Merge pull request #24206 from mnjul/bug_1063461_kb_manager_cleanup

Bug 1063461 - de-xfail keyboard_manager.js and annotate private members. r=rudyl

- Remove keyboard_manager.js from js linter xfail list (and make it jshint-compliant)
- Annotate private members of keyboard_manager.js with underscore-prefix