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
Change-Id: I7991fafe4978485354663f091f4d78a0cc73ba26
3 files changed