fix(Android): guard against re-adding an attached fragment in ScreenStack.onUpdate

[JacquesLeupin]

Description

Fixes an IllegalStateException: Fragment already added: ScreenStackFragment crash in ScreenStack.onUpdate.

When the top screen is translucent and visibleBottom's fragment is not added, the re-attach loop adds every wrapper from visibleBottom upward without checking whether it is already attached. If a fragment above visibleBottom is still attached — we hit this in production after background/resume cycles with a transparentModal on top and enableFreeze(true) — the add throws on commit.

Closes #4156.

Changes

• Skip wrappers whose fragment is already added in the re-attach loop, mirroring the !newTop.fragment.isAdded guard in the sibling branch below.
• Wrappers in dismissedWrappers are exempt from the skip: their removal is queued earlier in the same transaction, so remove + add is the existing (legal) re-attach path and behavior there is unchanged.

The guard only changes behavior in states that currently throw — for any wrapper that isn't attached, the transaction is identical to before.

Test plan

• No deterministic repro, it's a race around fragment attachment state (details in Crash on Android: "Fragment already added" in ScreenStack.onUpdate with transparentModal after background/resume #4156).
• :react-native-screens:compileDebugKotlin passes with the change.

Checklist

• Included code example that can be used to test this change.
• Ensured that CI passes