Describe requested visibilities in public types (6/n: clean up)

This CL
- removes unused files and imports,
- refines WindowInsets.Type#toString(int),
- reuses WindowInsets.Type#toString(int) in ControllerActivity, and
- refines RemoteInsetsControlTarget#isRequestedVisible.

Bug: 253420890
Bug: 234093736
Test: Open Window Insets Controller and see if ControllableInsetsTypes
      displays expected strings.
Test: Execute `adb shell dumpsys window all` and see if the strings of
      "Requested non-default-visibility types" are as expected.
Change-Id: I2aa041096db9880a32281fa684312c1f43d1c4a3