Skip unknown check in bootclasspath_fragment (source version) if prebuilt version is in use.

Following aosp/2822531, when MediaProvider prebuilt is enabled, framework-pdf (new jar) will be removed from PRODUCT_APEX_BOOT_JARS. Subsequently there will be an error around the inconsistency (unknown) between bootclasspath_fragment (source version of MP with framework-pdf in its content) and PRODUCT_APEX_BOOT_JARS (framework-pdf removed). Since the source version of bootclasspath_fragment is not in use, we should ignore this check. See more detail in go/stale-mainline-prebuilts (issue from changes in framework/service jars in existing apexes).

Bug: 304719212
Test: lunch cf_x86_phone-next-userdebug & m
Test: lunch cf_x86_phone-trunk-userdebug & m
Change-Id: I0f62277a9e65522bf5a4ea1ae33c166f996c37d5