Rename EXPERIMENTAL_USE_OPENJDK9 to EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9.

The current EXPERIMENTAL_USE_OPENJDK9 environment variable is a legacy
from when the 8 -> 9 toolchain upgrade was happening. That migration
is done and the variable only affects the language level, so it should
have a name that reflects that. (The current situation would be
especially confusing if we started a 9 -> 11 toolchain upgrade,
presumably controlled by a variable like EXPERIMENTAL_USE_OPENJDK11,
since the two settings look incompatible but are actually orthogonal.)

The current variable historically allowed a value or "1.8" which meant
"use the OpenJDK 9 toolchain but target language level 8". That value
no longer has any meaning and the new variable doesn't allow it.

Bug: 131678633
Test: `make` with `EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=true`
Test: `javap -v $OUT_DIR/soong/.intermediates/libcore/core-oj/android_common/javac/classes/java/util/List.class | grep 'major version'` shows 53
Test: Audit all mentions of the old string here and downstream
Change-Id: Idad808c7f07913baba1a777627322d5452dabcef