Build: Experimental option to default to -source 1.9 -target 1.9

Enabled via:

export EXPERIMENTAL_USE_OPENJDK9=true

Other nonempty values of EXPERIMENTAL_USE_OPENJDK9 will
allow OpenJDK 9 toolchains, but will still default to
-source 1.8 and -target 1.8.

Note that -source 1.9 and -target 1.9 does not currently
successfully build.

Test: Treehugger.
Bug: 38177569

Experimental flag to set LOCAL_JAVA_LANGUAGE_VERSION := 1.9

Change-Id: I9eb881b3fbd1806984a132f6da7b5a4cc6612247