Allow non-resizable apps in split-screen (8/n)

Refactor SizeCompatModeActivityController to WM Shell. The old one will
be removed in separate cl as there are some dependencies in other
packages.

This cl is mainly for refactor. Major change will come after.
Besides refactor, also changed the mActiveButtons key to TaskId, because
launcher Task will also trigger onTaskInfoChanged, but should not remove
the restart button on the same display.

Bug: 176061101
Bug: 178327644
Test: manually verify that the size compat restart button works as usual
Test: atest WMShellUnitTests:SizeCompatUIControllerTest
Test: atest WmTests:SizeCompatTests
Change-Id: I9143e079a1a945b76c3c56596976dd6ad2802897