AAPT2: Fix pseudolocalization (again)

Pseudolocalization didn't properly handle spans in
strings like "<small><small>Hello</small></small>".

The spans would be identical and when doing range checks
only one of them would be updated.

Switched to a more robust way of extracting the relevant
chunks of a styled string. This uses a stack, which is more
in line with the real representation in XML.

Bug: 34088357
Test: make aapt2_tests
Change-Id: Ia4e4501713e688c96a89e26e4e2b1384f4cd3889