Improve accelerometer-based orientation sensing.

Improve accelerometer-based orientation sensing.

There were three main complains about orientation sensing:
* Switching to landscape when putting a device down on a table (or picking it up)
* Changing orientation due to road bumps or vehicle vibrations while in a car dock
* Switching to upside-down too easily

This change includes three primary enhancements.

First, we run the accelerometer output through a lowpass filter before considering its orientation.  This avoids glitches due to brief phone movement, particularly when the phone hits a table.  The filter uses a very low default time constant of 200ms to retain responsiveness (note the samping period is ~200ms, so the effect of this filtering is pretty mild).  At tilt angles beyond 45 degrees, however, we increase the time constant to 600ms, which helps greatly with avoiding glitches picking the phone up from a table.  It does introduce some sluggishness when rotating while the phone is tilted back, i.e. being used in one's lap.

It's also worth mentioning that the accelerometer output on Sapphire appears to be pre-lowpass-filtered with a time constant of around 500ms, making this less necessary on that device, but the added effect doesn't noticeably degrade user experience in my opinion.

Second, we check the magnitude of the raw accelerometer output.  If it deviates from the strength of gravity by more than one m/s^2, we distrust the data, since that implies the device is under external acceleration and the sensor data doesn't accurately reflect orientation.  This helps avoid glitches due to shocks and vibrations, as in the car dock scenario.  However, rather than ignore the data entirely, we filter it with a very high time constant (5 sec).  As a result, if the device is rotated while vibrating, even if we never pick up a clean sample, we will eventually detect the orientation switch.  Of course, with a sampling period of 200ms, we're prone to aliasing, but that seems like a highly unlikely corner case.

Third, we restrict transitions to upside-down orientation to a much narrower range, both in terms of orientation and tilt.  This should prevent upside-down mode from activating in most cases where it's not desired.

I also updated a lot of stale documentation, added a lot of documentation, and cleaned up a lot of the code, so as to make this (often subtle) code as transparent as possible.