Allow java targets to support 1.8 source / target

This change enables build rules to specify:

LOCAL_JAVA_LANGUAGE_VERSION := 1.8

to enable -source 1.8 -target 1.8 for javac and
equivalent flags for Jack.

Bug: 26753820
(cherry-picked from commit cdfbe4a8)

Change-Id: I361c99dd599e7b4a041f02c9562e461da2b0502e