Remove Android's white background color.

[timbotimbo]

Description

This removes the white background set in Kotlin, which allows us to experiment with a transparent UnityWidget.
iOS doesn't need any change as this already works.

This should not affect most users, because to see any change you need to both manually configure Unity and adjust the android theme.

See this example of a video widget playing behind Unity to get an idea of the possible results.
(The video is started on a small delay to show the black flash, which is a flutter bug with AndroidView.)

Screen_Recording_20221119-193603.mp4

See #190 (comment) for more details.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[fiercepark]

@timbotimbo How did you get the transparent unity background color on android? I set the camera color to black with transparency 0 as well as "render over native ui" and this does not work for me, it still shows the black background.

[timbotimbo]

@fiercepark
Check step 3 of the original comment here #190 (comment).

I needed to adjust the android styles.xml to allow transparency.

[hehe605715432]

hey!
it worked well with @timbotimbo instructed at first.
However, the project stops working after enable HDR.
It works on unity version 2021.3.29f1c1.

I would really appreciate any help :)