Fix layoutlib compilation with OpenJDK 9 toolchain.

layoutlib needs to support compilation and running (on host)
under OpenJDK 8. But before this CL, layoutlib did not specify
LOCAL_JAVA_LANGUAGE_VERSION so it was targeting whatever .class
file version and source language level was the toolchain's
default.

Commits f89deaeb and
42e20816 introduced imports
from sun.* that are not visible under OpenJDK 9 -target 1.9,
breaking such builds.

This CL fixes the build by explicitly setting

LOCAL_JAVA_LANGUAGE_VERSION := 1.8

which means that layoutlib is compiled with language level 8
and v52 .class files even when the OpenJDK 9 build toolchain
is used.

Bug: 122643370
Test: EXPERIMENTAL_USE_OPENJDK9=true make layoutlib

Change-Id: Ia37c6bf16daf1e46e8eed79a365fda8b705cc55f