regex-instance: HalInterface: add regex API and hide public fields

<regex-instance> is added under an <interface> tag,
which is represented by an HalInterface object. Add
API for looping over all <regex-instance>s / <instance>s
under it.

With this API, HalInterface can hide its public fields.

Bug: 73738616
Test: libvintf_test
Test: vintf_object_test
Test: vts_treble_vintf_test

Change-Id: Ia072dd7e946c28789797db8a46fd968bc7872dca
12 files changed