Hardcode "-source 1.8" for Doclava.

Due to various bugs hidden deeply within Doclava, running with
 "-source 1.9" is always failed for it.

So switch to "-source 1.8" for now. This includes the case of running the
combination of Metalava + Doclava. Once we have new documentation tool
in system, we can full deprecate Doclava.

Test: USE_R8=true EXPERIMENTAL_USE_OPENJDK9=true m -j core-docs
Bug: b/70351683
Change-Id: I9553f88d0dd9bc263a249c49075c0b931b5d4927