Rename PRODUCT_UPDATABLE_SYSTEM_SERVER_JARS to PRODUCT_APEX_SYSTEM_SERVER_JARS.

Regardless of an "updatable" property, list all apex jars in the same
variable. This is less confusing for devs and matches the pattern with
PRODUCT_APEX_BOOT_JARS.

Bug: 191127295
Test: atest CtsClasspathsTestCases
Change-Id: I3b12f26237636f4271cb000480928b3ce1c2e62f
Merged-In: I3b12f26237636f4271cb000480928b3ce1c2e62f