Add TaskSnapshot reference when used for Content suggestion.

Add reference, so the hardware buffer of the snapshot won't get release
too early.

Flag: com.android.window.flags.release_snapshot_aggressively
Bug: 238206323
Test: enter recents to check content suggestion several times without
seeing crash.

Change-Id: I622e471bfc0f40cf97653dd3ace56e29b49185f7