Reduce memory usage for drawing app theme snapshot.

Since the content of the app theme snapshot is mostly a monochrome
image, drawing it in a smaller size should be a reasonable solution
to prevent memory waste.

Flag: com.android.window.flags.reduce_task_snapshot_memory_usage
Bug: 238206323
Test: verify the memory usage of app theme snapshot become much
smaller, and there is no obvious difference on recents animation.

Change-Id: Ibc74938adbf323e4bf703135db17aaf8f7433f72