Don't dexpreopt APEX variant modules

Java modules that are APEX variant won't be dexpreopted.

After applying this CL,
- $(ANDROID_PRODUCT_OUT)/apex/com.android.appsearch/javalib/service-appsearch.jar
- $(ANDROID_PRODUCT_OUT)/apex/com.android.cronet/javalib/org.chromium.net.cronet.jar
- $(ANDROID_PRODUCT_OUT)/apex/com.android.os.statsd/javalib/service-statsd.jar
- $(ANDROID_PRODUCT_OUT)/apex/com.android.telephony/app/StkLib/StkLib.apk

above four files are not preopted, thus the dex files aren't uncompressed.

FIle size of JAR/APK is (before / after):
- service-appsearch.jar:        697594 / 273354
- org.chromium.net.cronet.jar:  607206 / 239990
- service-statsd.jar:           90806 / 41857
- StkLib.apk:                   6424332 / 2942719
File size of APEX is (before / after):
- com.android.appsearch.apex:   854851 / 432963
- com.android.cronet.apex:      4982739 / 4618195
- com.android.os.statsd.apex:   322319 / 322319
- com.android.telephony.apex:   6596717 / 3111021

Bug: 146984232
Test: lunch aosp_cf_x86_phone-userdebug; m
Test: Build aosp_cf_x86_phone with and w/o this CL and compare artifacts.
      After applying this CL, javalibs and apps embedded inside APEX
      were not preopted, thus dex file are not uncompressed.

Change-Id: I296173bc29d0df0c4d5cc59e4b070e5c4d3b50d5