Extend bounds computation to support horizontal layout

This CL introduces SplitLayout and update bounds computation methods
to support horizontal layout. It also introduces #computeSplitLayout
to compute the current split layout, which always returns the default
splitLayout currently. #computeSplitLayout will extend to consider
SplitLayoutCalculator in later CL.

fixes: 241042437
Bug: 241043028
Bug: 207494880
Test: atest SplitPresenterTest
Change-Id: I037a2b2fa38d05c4fe97e4734cc044148ad15806