Use build flag to control the apex boot jars to be excluded.

Different prebuilts might have different contents and thus, different
boot jars to be removed from the existing apex boot jar lists e.g. next
using U prebuilts would need to remove framework-pdf while ap31 with V
prebuilts must not remove such. The existing mechanism will always
remove framework-pdf whenever prebuilts are enabled regardless of
release config, which is not correct in context of multiple prebuilt
support.

Ignore-AOSP-First: for successful build, we need to submit this togther with ag/26332775. this cl will get cp'ed into aosp later on
Bug: 327022391
Test: build device in next and trunk
Change-Id: I9949fb7075ab6c207a292f13fd75e010a4958f87