vndk-def: Ignore dex strings w/ invalid surrogates

The previous CL silently ignores all dex strings that cannot be decoded
with Modified UTF-8 decoder.  However, bad unicode encodings are
usually a good indicator that the dex file format is changed.  Thus,
this commit separates invalid surrogates as special cases and only
ignore those dex strings.  Exceptions for other invalid encodings are
propagated after this CL.

Bug: 110450167
Test: ./tests/run.py
Change-Id: I61fd754eab76f0b9eb3c58ac7f48321258e46514