Rename themed icon repository to icon style repository (1/2)

Rename and move themed icon classes so it can be repurposed for general
icon theming.

Flag: com.android.systemui.shared.extendible_theme_manager
Bug: 397782741
Test: manually verified with flag on and off
Change-Id: I494c0efe0b0ac0a89d504ed4f0ca1a0e104d2524