Fix incorrect emoji display in messages and reactions.

[SanyamSuyal]

Emoji variations were incrementing the sprite index incorrectly, causing wrong emojis to display when mixed with text or used as reactions.

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Description

Problem:
Emoji variations (such as skin tone modifiers) were incorrectly incrementing the sprite sheet index for each variation. This caused wrong emojis to display when mixed with text or used as reactions.

Root Cause:
The sprite sheet contains only one image per base emoji, but the code in EmojiSource.kt was creating separate indices for each variation (default + skin tones), causing an index mismatch between the emoji tree and sprite positions.

Solution:
Modified the emoji tree building logic to capture the index once per emoji and share it across all variations, ensuring all variations reference the same sprite sheet position.

Testing:

• Code review confirms the fix aligns with the sprite sheet architecture

Changes:

• app/src/main/java/org/thoughtcrime/securesms/emoji/EmojiSource.kt (2 lines changed)

Fixes #14361