Use variable reaction time to uncovering of the proximity sensor

Previously 500ms delay was always used. While this makes stray touch
events less likely to occur when the phone is held at ear, it also
delays for example triggering of lpm display states when the phone
is removed from a pocket.

Now shorter delay is used unless there is an active call.
Add support for fixed time brightness fading

Previously display brightness fading was done using constant velocity
or in constant time without considering the type of transition that
was taking place. This made large changes like changing brightness
settings or display dimming take a long time and/or easy to miss.

Now separate fade durations are used when:
- blanking screen
- unblanking screen
- dimming screen
- other display state changes
- adjusting to ambient light / brightness setting changes

The values can be changed via mcetool options
      --set-brightness-fade-def=<msecs>
      --set-brightness-fade-dim=<msecs>
      --set-brightness-fade-als=<msecs>
      --set-brightness-fade-blank=<msecs>
      --set-brightness-fade-unblank=<msecs>

Setting a value to zero disables fading and immediate transition is made.

Also prefers direct sysfs manipulation for brightness control over using
android hal via libhybris since the latter introduces some overhead and
does not provide a way to read current backlight brightness level.

[mce] Add support for fixed time brightness fading. Fixes JB#21713
Rename new brightness fading GConf settings for clarity

Avoid short forms, add unit suffixes and brief explanations.
Rename old display GConf settings

Remove superfluous "path" information from macro names and comments.
Actual key names are not changed for the sake of backwards compatibility.
Rename mdy_brightness_set_fade_target_* functions for clarity

Avoid short forms.
Add unit suffixes to fade duration variables

Also break long lines and remove excess whitespace.
Do not use fade-to-black during active calls

Needed to make stray touch input events from ear/chin less likely
when placing the phone at ear.

Also clarify why fade-to-black length should be kept short.
Clarify use and implementation of priority boosting during fading
Fix typo
Use rounding instead of truncating when calculating brightness fade value
Use helper function for common fade timer cleanup tasks
Explain why als based fading is not done during diplay state change
Allow faster fade to supersede existing one

If both ongoing and freshly requested fade have the same target level,
but the latter would finish before the former, reprogram the fade
timer.
Fix typos and whitespace in comment
Merge pull request #197 from spiiroin/brightness_fading

Add support for fixed time brightness fading