Rename PRODUCT_UPDATABLE_BOOT_JARS to PRODUCT_APEX_BOOT_JARS.

Regardless of an "updatable" property of individual, list all apex boot
jars in the same variable. This is less confusing for devs, especially
since they shouldn't care about things like boot images.

Bug: 191127295
Test: atest CtsClasspathsTestCases
Change-Id: I0a559db462d1e1f67003ac54d1e27a89110d802a