[linear-gradient] fix crash when r8 enabled (#21580)

# Why

fixes #21562

# How

the code here will be stripped by R8: https://github.com/expo/expo/blob/4d234bbfb32b66b3944a0e652eda1e6c6728705

[linear-gradient] fix crash when r8 enabled (#21580)

# Why

fixes #21562

# How

the code here will be stripped by R8: https://github.com/expo/expo/blob/4d234bbfb32b66b3944a0e652eda1e6c67287059/packages/expo-linear-gradient/android/src/main/java/expo/modules/lineargradient/LinearGradientView.java#L25-L27 and then our `ViewDefinitionBuilder` cannot find the primary constructor. this pr adds the keep annotation.

i didn't find a way to fix it systematically from expo-modules-core. let's address case by case and expo-linear-gradient first.


# Test Plan

https://github.com/GaelCO/repro_linearGradient with patch-package for the fix

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