Sort PRODUCT_APEX_SYSTEM_SERVER_JARS automatically.

This is needed because product makefiles may need to add their own
system server jars to this variable, 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: atest art_standalone_dexpreopt_tests:art_standalone_dexpreopt_tests.DexpreoptTest#ForSystemServer
Change-Id: I80dbf555c395c92539c506d3d8b1f2f101a58bce