*** Original commit ***
Remove `@Nullable` from `MediaSource.Factory` setters
The null-behaviour of these methods creates a minimization footgun,
because **any** call to these setters will prevent R8 from removing
the default implementation (even if it's never used by the app) - this
is because R8 can't tell the default implementation is only used if the
parameter is `null`.
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PiperOrigin-RevId: 450410833
