[SB][Chips] Add tint for decorative icon

This change adds a tint color to the decorative icon model for ongoing activity chips. This allows the icon's color to be customized independently of the background.

Bug: 411716730
Test: atest SystemUIGoogleScreenshotTests
Flag: com.google.android.systemui.assistant_session_chip_custom_treatment
Change-Id: I2dfb01986ebb00d97fccec5697bc828a93f74b56