Rename dexpreopt.config field to bring it in sync with Make.

Different names of the same field in Make and in Soong resulted in
the field being dropped when unmarshalling the JSON from Make to Soong.
As a result, the list of optional used libraries for Make modules was
always empty.

Test: lunch aosp_cf_x86_phone-userdebug && m
Test: Cherry-pick in internal master and observe that the logcat error
    "ClassLoaderContext shared library size mismatch" for GoogleDialer
    has disappeared:
    terminal-1 $ lunch cf_x86_phone-userdebug && m && launch_cvd
    terminal-2 $ adb wait-for-device && adb root \
        && adb logcat | grep 'size mismatch'
    (nothing after the change, one error message before the change)

Bug: 132357300
Change-Id: I57820646fa7c1d8af01b2cbc8a3521da450719c0