Update icon background color of the private space widget tile

This icon background should be the same as the web icon backgrounds in Toast which is: materialColorSurfaceContainerLowest

bug: 320562455
Test: Manual - photos:
after private space qstile quicklaunchable dark theme: https://drive.google.com/file/d/1UZUxOdyEMEyLmPhnuPWJ085gQHrGgUdC/view?usp=sharing
after private space qstile nonquicklaunchable dark theme: https://drive.google.com/file/d/1UOoR0VHxdIIvMFfvhc_5MwHsg40wURA3/view?usp=sharing
after private space qstile quicklaunchable light theme: https://drive.google.com/file/d/1Ub1Nra_5rMlWuciI_a6yJM48EL6IdEsp/view?usp=sharing
after private space qstile nonquicklaunchable light theme: https://drive.google.com/file/d/1UidMqcHI2rXTtwmwIRXkGiKJkaeP8ubi/view?usp=sharing
Flag: com.google.android.apps.nexuslauncher.enable_inject_private_space_tile

Change-Id: Id118a6cc69d091b216c49b7fe778d8b6c84966b2