Preserve <uses-library> order in dexpreopt.config files.

Library order is important because it is used to construct class loader
context, which is then written into OAT/ODEX files and chacked against
class loader context constructed by PackageManager on the device. If the
orders are different, dexpreopted code is rejected.

Soong avoids using Go maps for class loader context representation
precisely for that reason. However, for the modules defined in makefiles
dexpreopt configs were serialized to JSON and unmarshaled to Go maps,
which resulted in wrong order of libraries. This CL changes Go
representation of class loader contexts imported from JSON and makes the
order stable.

Bug: 132357300
Test: lunch cf_x86_64_phone-userdebug && m && launch_cvd \
      adb wait-for-device && adb root && adb logcat \
      | grep -E 'ClassLoaderContext [a-z ]+ mismatch'
      # empty grep output, no errors
Change-Id: I15f51617f9573c0bbcb324cf2592daf719cad586