Add BUILD_BROKEN_DONT_CHECK_SYSTEMSDK

https://android-review.git.corp.google.com/q/topic:limit_systemsdk
introduced a new check for preventing the use of system SDKs above 34
from Java modules in the vendor partition.

As this may break some unprepared targets, introduce
BUILD_BROKEN_DONT_CHECK_SYSTEMSDK as a temporary escape hatch.

This flag will be deleted eventually.

Bug: 314011075
Test: Add BUILD_BROKEN_DONT_CHECK_SYSTEMSDK := true to BoardConfig.mk
Change-Id: Ie70ce428ec3df8580099efd99e5c7c9c4f9daecb