Revert "Rasterizer is initialized with an external view embedder (#22348)"

[cbracken]

Description

This reverts commit 0d5f2e9.

Due to merge conflicts, this also reverts #22275:

Revert "Do not involve external_view_embedder in submit frame process if threads are not merged. (#22275)"

This reverts commit 016fbde.

Related Issues

flutter/flutter#69983 [tool_crash] RPCError

Tests

n/a

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[cyanglaz]

LGTM