Deduplicate `classLoaderContexts` field between two structs.

This field is provided by the `dexpreopter` struct, which is a part of
`java.Module` and also had an identially named field. This created
confusion when the latter field was not properly copied into the former,
which resulted in not propagating class loader context, e.g. for static
library "androidx.preference_preference". This didn't cause class loader
context mismatch errors at boot previously, because the library didn't
have any uses-library dependencies before a recent prebuilt update.

Bug: 214255490
Test: lunch aosp_cf_x86_64_phone-userdebug && m && launch_cvd \
    && adb wait-for-device && adb root \
    && adb logcat | grep -E 'ClassLoaderContext [a-z ]+ mismatch'
    # empty output, no errors at boot
Change-Id: Ib818c5d2934d28817bb7a04b6114ae8b82a5c04d