Additional interfaces for main/side stage split-screen model

Add some more interfaces APIs for manipulating main/side stage
split-screen.
Also, renamed pinTask/unpinTask to moveToSideStage/removeFromSideStage
for consistency.

Test: Command line
Bug: 176061049
Change-Id: Ic5b909c42e027e6f4809931c6dc92af1bf79b050