SystemUI: add left and right virtual buttons while typing

Adapt for cm-12.0.

[cyanogen] refactor for cm-14.0
[LuK1337] update for 8.1

Squashes the following commits:

    Fix side button visibility tracking.

    Previously one could e.g. get legacy menu button being visible in
    certain scenarios, like this one:
    - Open legacy app, e.g. CatLog
    - Invoke IME
    - Lock screen
    - Unlock screen
    - Result: Menu button visible in keyguard

    Change-Id: Ia796f73ec995ea1cc34d38f8aefdbccda7be7565

    SystemUI: fix up navigation cursor keyevent flags

    Volume cursor controls use slightly different flags to keep the cursor
    within the field. Match them in the SystemUI for consistent (and better)
    behavior.

    Change-Id: I56d597f111ec611b0ca24a10f30575401fa76d18
Signed-off-by: Roman Birg <roman@cyngn.com>

    SystemUI: fix up initial cursor down flags on nav bar

    We were using the regular flags for the initial ACTION_DOWN event so the
    FLAG_KEEP_TOUCH_MODE was not being passed and allowed the cursor to jump
    out of bounds slightly.

    Change-Id: I5b4d2a5e5e0ce562985fcc3b7b6fac8b4abc49da
Signed-off-by: Roman Birg <roman@cyngn.com>

Change-Id: Iea446443d33ff305765e47c86523b619676116ac