Support PTT, SYM and FN keys

[rfmejia]

Enable the use and customization of PTT, SYM and FN keys. AFAIK these keys have no function on the Titan Pocket (unsure about other Titan devices). I can't tell if the key event is not emitted correctly (or even at all) in the current release.

Some examples of where they were useful in the stock ROM:

• Short/long/double press PTT to launch apps, turn on do not disurb or the flashlight, etc.
• SYM key with Blackberry keyboard opens custom symbols UI, or SYM+key for quick symbol insertion
• Remap FN to Ctrl for traditional shortcuts like Ctrl+a, Ctrl+c, etc.