Support dragging divider bar of app-pair to resize the splits

Update SplitLayout and DividerView to support dragging divider bar and
make them more generic to be able to share among different split impl.

Bug: 172704238
Test: manual check the behavior of divider bar
Test: atest WMShellUnitTests
Change-Id: I16ed1fd86bdff4d8544c5787c95315c59055ad04
13 files changed