Window Manager Flag Migration (6/n)

Refactor the insets APIs:
- Remove top/side in the names of types
- Add a type for the caption bar
- Rename InsetSide to InternalInsetsSide which is a enumeration, so that
  we can have a public @InsetsSide which is a bit-wise variable
- Fix the naming about insets: inset --> insets

Bug: 118118435
Test: atest InsetsSourceProviderTest InsetsStateControllerTest
            InsetsPolicyTest WindowStateTests CommandQueueTest
            RegisterStatusBarResultTest InsetsFlagsTest
            LightBarControllerTest RegisterStatusBarResultTest

Change-Id: I27a066299eea9927ce61f6cb5d489ad79bca42ba