Sort PRODUCT_APEX_BOOT_JARS automatically.

This is a follow-up CL for https://r.android.com/1900565.

This is needed because product makefiles may need to add their own
APEX boot jars, and they cannot insert them in the correct alphabetical
order on the list. So it needs to be sorted automatically. Do this
after processing all product makefiles and populating the variable,
but before passing it to Soong.

Bug: 207474880
Test: lunch aosp_cf_x86_64_phone-userdebug && launch_cvd
      # cuttlefish device boots successfully
Change-Id: I86b58ef8aeac595e401d872f59a2936a67bedeec