Build java_sdk_library stubs with language level 8

This is in line with the main android stubs (android.jar). I discovered
the difference since we want to merge some java_sdk_libraries into the
android.jar files, but I believe the rationale for compiling it with
version 8 applies to all stubs (we don't want level 9-language features
because some tooling may be stuck with version 8).

Bug: 144149403
Test: presubmit (checkbuild)
Change-Id: I9439ed516dda68ef516ff08d1c6045de3d3c1c0e