[SysUI][Floaty] Move power button pressed and long pressed to StateFlow

Power button down event is subscribed by multiple consumers, moving to
StateFlow ensures that all consumers receive same updated latest value.

Bug: 399263897
Flag: com.android.systemui.shared.enable_lpp_squeeze_effect
Test: atest KeyEventRepositoryTest
Change-Id: I0fb775579425af69171db468e19dbb7ddd47c39d