Use Otsu's thresholding get better transition effect.

We can gt more appreciate threshold for the image wallpaper transition
by Otsu's thresholding algorithm. In addition to Otsu's algorithm, we
also limit the max threshold.

Bug: 130360362
Test: Set wallpaper in "Art collection" category.
Test: Press power to suspend / resume device.
Test: Observe the transition, it is nice now.
Test: Observe wallpaper in "Solid Color" category.
Test: Observe wallpapers in any other categories.
Change-Id: I2d6a4e9d19b8a8f1636bd2233e66bdba6e92909c