kotlinc: Always pass -jvm-target 1.8

AOSP's current version of kotlinc only supports -jvm-target
1.8 or 1.6. Previously, we were passing the javaVersion
(the value passed to javac -target) but that would fail if
javaVersion was eg. 1.9.

This CL changes the kotlinc invocation to always pass
-jvm-target 1.8, regardless of javaVersion.

Bug: 69160377
Test: Treehugger
Change-Id: Ib12a8c2afbe908319513861a5380e70bf09d0be0