Switch to an OpenJDK 11 toolchain by default.

Prior to this change, the default was to use and OpenJDK 9 toolchain,
with the EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN=true environment
variable available to opt into OpenJDK 11. After this change, the
default is to use OpenJDK 11, with
EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN=false available to opt out.

This change affects:
 - The version of javac used.
 - The version of javadoc used.
 - The version of other tools used during the build process (e.g. jar,
   jmod, and jlink).
 - For Java bytecode executed on the host, the version of the java
   executable used, and so the versions of the JVM and the OpenJDK
   libraries in the system module (or bootclasspath).

This change does not affect:
 - The Java language level, i.e. the version of Java expected in
   source code (the -source option to javac) or the version of Java
   bytecode produced (the -target option to javac).
 - Anything to do with code execution on the device.

Bug: 131683177
Test: make java droid docs cts tests
Test: zcat out/verbose.log.gz | grep 'prebuilts/jdk/jdk[0-9]*'
Test: make RunBluetoothRoboTests RunCarSettingsLibRoboTests
Test: cts-tradefed help
Test: atest CtsHostTzDataTests
Test: atest CtsLibcoreTestCases
Change-Id: Iaae89ef25c92ec099575c026fc50e41da4e143e8