Use the current java version for -jvm-target

This is a partial revert of aosp/541879.
In that cl, the kotlin jvm version was always
set to 1.8 because that's what the kotlin
complier supported at the time, but now
it supports more versions:

$ ./external/kotlinc/bin/kotlinc -jvm-target foo
error: unknown JVM target version: foo
Supported versions: 1.6, 1.8, 9, 10, 11, 12, 13, 14, 15, 16, 17

Bug: 69160377
Bug: 236431222
Test: Treehugger
Change-Id: I273e0b4d1f484013889e17c60bc1b299a3f975a1