check-flagged-apis: make interfaces inherit from java/lang/Object

When parsing API signature files, check-flagged-apis relies on
ClassItem.superClass to get the parent class of a class or interface.
That method always returns null for interfaces.

When generating api-versions.xml, metalava marks interface classes as
inheriting from java/lang/Object:

  <class name="android/os/Parcelable" since="1">
    <extends name="java/lang/Object"/>
    [...]
  </class>

This confuses check-flagged-apis when comparing data parsed from both
sources, as the symbol signatures will be identical, but the superclass
entries differ. Work around this by explicitly marking all interfaces
as inheriting from java/lang/Object when parsing API signature files.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: Icbb8f7d4c3d4232a083289a778b347e33a0856ab