RFR: 8341184: Clean up the interaction between the launcher native code and the LauncherHelper

[Jaikiran Pai]

Can I please get a review of this change, which simplifies the interaction between the `java` launcher's native code with the `sun.launcher.LauncherHelper`? 

As noted in https://bugs.openjdk.org/browse/JDK-8341184, this proposed change reduces the back and forth between the launcher's native code and the `LauncherHelper` class. This also removes the additional reflective lookups from the native code after the main class and main method have been determined by the `LauncherHelper`. 

Although this is a clean up of the code, the changes in the `LauncherHelper` to return a `MainEntry` have been done in a way to facilitate additional upcoming changes in this area, which propose to get rid of the JAR manifest parsing from the launcher's native code.

No new tests have been added. Existing tests in tier1, tier2 and tier3 continue to pass.

-------------

Commit messages:
 - 8341184: Clean up the interaction between the launcher native code and the LauncherHelper

Changes: https://git.openjdk.org/jdk/pull/21256/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341184
  Stats: 367 lines in 2 files changed: 105 ins; 172 del; 90 mod
  Patch: https://git.openjdk.org/jdk/pull/21256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21256/head:pull/21256

PR: https://git.openjdk.org/jdk/pull/21256