Add logs when starting/finishing a transition

This CL adds simple logs when starting or finishing a transition. These
logs should be useful but also they shouldn't spam logcat too much,
which is why they are always enabled.

Bug: 373799480
Test: Manual, saw logs when performing transitions
Flag: com.android.systemui.scene_container
Change-Id: I0440b00de0dccfe6628b316d890cab48d4a94a40