vndk-def: DexFileReader should return ModifiedUTF8

This commit lifts the decoding logic from DexFileReader.  After this
commit, DexFileReader simply returns modified UTF-8 so that the user can
decide whether to decode it or not.

Bug: 110450167
Test: ./tests/run.py
Change-Id: I71bdc5e20553a17e9eab3eac597648e397e99730
2 files changed