Cleanup parse_xml.cpp

* add appendChildren (that corresponds to parseChildren)
* add appendTextElements and parseTextElements
* Detect duplicated manifest.hal and compatibility-matrix.hal entry
* Put more error messages

Bug: 34620633
Test: libvintf_test
Change-Id: Ice0e99bc9faca7bc61cddd5a2a27c16750fb88a7
2 files changed