Allow passing the initial value for FlexibleSheetValue

[sergionettl]

Is your feature request related to a problem?

Currently the function that allows settings it is hidden as a internal.

Describe the solution you'd like:

Add an extra paramter initialValue : FlexibleSheetValue to the public rememberFlexibleBottomSheetState

Describe alternatives you've considered:

Doing myself the function + remember + saver