Align boot jar exclusion with apex_contributions

`RELEASE_APEX_BOOT_JARS_PREBUILT_EXCLUDED_LIST` exists to flag apex boot
jars that may exist in source builds, but not in a google apex prebuilt
(a prebuilt might not have been generated yet). This is a build flag, and
exists in the <rel>.scl

It is feasible that certain products in a specific release config
never use apex prebuilts. Therefore, we need to selectively
apply `RELEASE_APEX_BOOT_JARS_PREBUILT_EXCLUDED_LIST` to products that
use apex prebuilts. At ToT, this is done via looking at
module_build_from_source_value of ANDROID soong config namespace, and
<apex>_module soong config namespace for every optional module.

<apex>_module soong config namespace was removed in
https://r.android.com/3056785. In preparation for removing
module_build_from_source, use
`PRODUCT_BUILD_IGNORE_APEX_CONTRIBUTION_CONTENTS` to determine if
prebuilts are being used.

Test: in internal, lunch barbet-ap2a-userdebug && get_build_var APEX_BOOT_JARS_EXCLUDED
com.android.mediaprovider:framework-pdf com.android.mediaprovider:framework-pdf-v
(this is the only release config where there is a delta between prebuilt
and source apex boot jars)

Test: in internal, lunch barbet-trunk_staging && get_build_var
APEX_BOOT_JARS_EXCLUDED
"" (empty string)

Change-Id: I981ad3a7767f3602a844c8d53eadfd1f2e5db420