Screenshot request processor

Inserts a preprocessing stage to screenshot request handling
to contain support for new features. This new branch is
guarded by the flag 'SCREENSHOT_REQUEST_PROCESSOR'.

To enable:

 adb shell cmd statusbar flag 1300 on

No other functional changes should be observed.

Bug: 231957192
Bug: 200294244
Test: atest RequestProcessorTest
Test: manual; enable flags, take screenshots

Change-Id: I2fb26f9bf30513199b7e6d5cb97f6ccf99fcc0eb