Support Dokka in Soong.

Metalava is supposed to treat all the args after
"--generate-documentation" as either Javadoc or Dokka commands, and it starts
 seperate process to invoke javadoc or java -jar dokka...

Dokka doesn't support --bootclasspath in its args, so treat all the
bootclasspath as classpath.

Also continue to refactor code to seperate Dokka runs from Javadoc or
Metalava.

Test: m -j metalava-dokka-core-docs
Bug: b/72394196
Change-Id: I0f0f3dd80cb2dbb53f19da8fa11ae0b1d92ac5d7