Allow non-resizable apps in split-screen (1/n)

1. Add a setting to enable this.
2. Allow non-resizable apps to be moved to split-screen when the setting
is on.

TODO:
1. Add a developer option for this
2. Fix SizeCompatMode when the non-resizable app is in split screen

Bug: 176061101
Test: adb shell settings put global enable_non_resizable_multi_window 1
Test: atest WmTests:ActivityRecordTests
Test: atest WmTests:WindowOrganizerTests
Test: atest WmTests:WindowManagerSettingsTests
Test: atest CtsWindowManagerDeviceTestCases:MultiWindowTests
Change-Id: Ife9e5e5848614f072efb3b23f71472561e00488c