Android: Add export option for custom theme attributes

[syntaxerror247]

• Regenerate the GodotAppMainTheme and GodotAppSplashTheme during Android export. Any manual changes to this style will be cleared and replaced with default theme attributes.

• Adds a new export option gradle_build/custom_theme_attributes for injecting custom theme attributes directly via the export window, avoiding the need to manually modify themes.xml.

It helps to avoid the issue I metioned in #106709 (comment).

According to the attributes shown in the image:

• The first pair adds <item name="android:windowIsTranslucent">true</item> to the GodotAppMainTheme.
• In the second pair key is prefixed with [splash], so it adds the attribute to the GodotAppSplashTheme.

[m4gr3d]

Similar logic needs to be applied to GodotAppSplashTheme as well. In my PR, the android:windowIsTranslucent attribute is added to both themes, and updated when transparency is enabled.

[m4gr3d]

To address that, we should be able to duplicate the same set of attributes across both themes. We can have different defaults for each themes, but the custom theme attributes should be applied to both themes.

[syntaxerror247]

We can re-generate it during export with only default attributes. GodotAppSplashTheme only handles the splash screen, I don't see any need for a custom attribute here.

[m4gr3d]

I would have agreed, except that my testing of the transparency PR showed that I needed to set android:windowIsTranslucent on both themes for it to take effect.

I suspect that because it's the first theme, the splash theme may set some properties that cannot be updated by the main theme.
And so to ensure that the custom theme attributes don't run into the same problem I did, they should be applied to both themes.

[syntaxerror247]

I'm not sure about duplicating all custom theme attributes. Some attributes in GodotAppSplashTheme might cause unexpected behaviour and user wouldn't have any way to avoid that without core changes or removing that attribute from both themes.

[syntaxerror247]

@m4gr3d I introduced a prefix for GodotAppSplashTheme attributes. If custom_theme_attributes has a key like [splash]android:windowIsTranslucent it would be added to the GodotAppSplashTheme.