Native camera warper libraries build improvements

Update modules/ml/doc/gradient_boosted_trees.rst

Environment variables support added to Android tree path;
Default config updated to use env variable.

This reverts commit 2e7656db.

The documentation is available online at http://opencv.org/documentation.html

removing Android tutorial 0

removing Android tutorial 0 since it has become irrelevant after moving to the new application framework

Sample renamed to CameraControl;
Picture taking added to show camera preview restart.

MatInfo_Size_Size.resizeDownLinear
MatInfo_Size_Size.resizeUpLinear
TestWarpPerspectiveNear_t.WarpPerspectiveNear
TestWarpPerspective.WarpPerspective

Patch #2323 applied

now invert 3x3 on "bad" matrices works well on Windows

fixed Bug #2581

GoodFeaturesToTrackDetector_GPU fails when no corners found

Increased time limits for perf tests

Android: always sort list of NDK toolchains

- Added Gain control
- Changed default acquisition mode to grayscale
- Fixed a signed/unsigned compare issue
- Implemented frame resizing
- Implemented automatic buffer resizing on frame resize
- Corrected camera initialization so that it attempts to put the camera in Mono8
  but does not fail if this doesn't succeed.

Order returned by CMake is unreliable but we need clang to go after gcc for
automatic toolchain selection.