Start preparing for release 0.7.

Adjusted app theme def so that the camera background appears black.
(The area appearing when the camera preview and the screen have
different aspect ratio, similar to letter boxes.)
Some cleanup of XML resource files, including list warning corrections.

Updated manifest to add more info.
Fixed a problem that on HTC phone a wrong camera preview size is
chosen.
On a way to solve a problem that on an HTC phone puzzle capture fails
with very high rate (more than 75%.)

Added some log output in CaptureActivity and added toasts to distinguish
cancel and failure (in MainActivity).
_Fixed_ an issue that an HTC phone fails to recognize puzzle frequently.

I adjusted image processing parameters (to adaptive thresholding) to
cover the HTC phone.  The surface cause of the issue was the difference
of the camera characteristics.  The real cause, however, is that I used
a fixed set of processing parameters.  I just adjusted the fixed
parameters, so I may need to adjust them again to cover new phones, as
long as I stick on the fixed set of parameters...
Fixed the last issue on an HTC phone.
Disabled an unfinished _improvement_ on OpenCV library loading.
Wrote release notes for 0.7.