Reproduce camera sound only from default display screenshot controller

This avoids duplicated sounds and file loading for external displays. The sound will play only once after this cl. Note that even before, despite the sound was playing twice, it was not distinguishable.

For now it's not possible to take the screenshot of a specified display only, so reproducing the sound only from the default display controller is enough.

Some additional classes have been added to make this testable.

+ use only one bg thread in all ScreenshotController instances.

Bug: 290910794
Bug: 296545112
Test: ScreenshotSoundControllerTest + Connected to an external display, took screenshot.
Change-Id: I49fcd8a0dc7009c28557dc81deb91e79d5228074