[Partial Screensharing] Add initial recents selector UI

Adds a recycler view that can show list of thumbnails
and app icons that could be loaded asynchronously.
It is hidden under a flag, so it won't be visible
on normal builds.

Current limitations which will be fixed in
the follow up CLs:
* It doesn't load real data
* Task thumbnail sizes are hardcoded and
  not cropped correctly
* No actions on thumbnail click

Screenshot demo: https://screenshot.googleplex.com/68vhXBNefCKHR2G.png

Test: atest com.android.systemui.mediaprojection.appselector.MediaProjectionAppSelectorControllerTest
Bug: 240924921
Change-Id: Ia3c685050656659ea9a79ffc90a91465c114e259

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