[packages] fix another gradle 8 warnings (#22609)

# Why

there are other gradle 8 build warnings:

```
> Task :expo:processReleaseManifest
package="expo.core" found in source AndroidManifest.x

[packages] fix another gradle 8 warnings (#22609)

# Why

there are other gradle 8 build warnings:

```
> Task :expo:processReleaseManifest
package="expo.core" found in source AndroidManifest.xml: /home/runner/work/expo/expo/packages/expo/android/src/main/AndroidManifest.xml.
Setting the namespace via a source AndroidManifest.xml's package attribute is deprecated.
Please instead set the namespace (or testNamespace) in the module's build.gradle file, as described here: https://developer.android.com/studio/build/configure-app-module#set-namespace
This migration can be done automatically using the AGP Upgrade Assistant, please refer to https://developer.android.com/studio/build/agp-upgrade-assistant for more information.
```

# How

migrate `package` in **AndroidManifest.xml** to `namespace` in **build.gradle**

# Test Plan

ci passed

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[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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