Prevent the hub from recomposing while opening a timer activity

Create a flow `isCommunalContentFlowFrozen` to pause the emission of
communalContent flow. The freeze only happens if we last transitioned
to hub, keyguard is occluded and isAbleToDream is false as a new activity
is about to show on top.

Bug: b/338052219
Flag: com.android.systemui.communal_hub
Test: manually add a timer via voice and the grid doesn't re-flow
Test: atest CommunalViewModelTest
Change-Id: Ic0bea11e848da6820e44404005397b161bd88a7e