Fix SnapshotRecord being overwritten when requesting a new one.

Because the key of StartingWindowRecordManager is taskId, there can
always add/remove activity snapshot with the same key. So before add a
new record, remove previous starting window record immediately.

Bug: 328563338
Test: trigger cross-activity animation aggressively
Change-Id: I9122d5046a48ad9f7b5987686dcf2d42617686d4
1 file changed