Cap LOCAL_JAVA_LANGUAGE_VERSION <= 1.8 for API versions <= 26.

The default LOCAL_JAVA_LANGUAGE_VERSION is normally 1.8, or 1.9
when EXPERIMENTAL_USE_OPENJDK9 is explicitly set to the
nondefault value of "true".

Since old Android API levels do not support later versions,
LOCAL_JAVA_LANGUAGE_VERSIONs is already capped at 1.7 for
APIs <= 23.

This CL further caps it at 1.8 for all released APIs (currently
<= 26) even when EXPERIMENTAL_USE_OPENJDK9=true.
This may need to be updated in future.

This CL only affects builds where EXPERIMENTAL_USE_OPENJDK9 is
set; it does not affect the default build.

Bug: 67454477
Test: Treehugger

Change-Id: I54ca72bdb09aeb4184d5bb3c51d881dcfc943839