[J2KT] Update visibilities in com.google.apps.xplat.collect.intervaltree to compile in J2KT with proper visibility mapping enabled.

Details:
- most `protected` methods and types were resticted to `package-private`, since they are accessed outside of subtype but are effectively internal,
- nested `Pair` class is converted to `package-protected` as it's exposed in `package-private` member
- `ComparableInterval` is converted to public, as it's extended by `public Interval` class.
PiperOrigin-RevId: 926677909

Author: Googler

transpiler/java/com/google/j2cl/transpiler/backend/kotlin/Visibilities.kt

@@ -31,6 +31,7 @@ private val ACTUAL_VISIBILITY_PACKAGE_PREFIXES: List<String> =
     "j2ktobjcweak.",
     // copybara:strip_begin
     // Packages which should be hidden in Open Source project.
+    "com.google.apps.xplat.collect.intervaltree.",
     // copybara:strip_end
   )