Check that API is up-to-date when building java_sdk_library

Previously, the checked in version of an API was only checked to make
sure it was up-to-date when running the checkapi target. This change
adds a validation dependency to the ninja rule that generates the API
from source so that up-to-date check is always performed every time
the API is generated. However, because it is a validation dependency
it does not lengthen the build's critical path.

Bug: 234113632
Test: echo > packages/modules/SdkExtensions/java/android/os/ext/api/current.txt
      m framework-sdkextension
      # Passes without this change even though the checked in API is
      # not up-to-date.
      # With this change the build fails reporting correctly that the
      # checked in API is not up-to-date.
Change-Id: I8e65cf716d94aecd61bd943f1485468664ad4a22