RFR: 8368333: [lworld] Add preview mode to ImageReader and JRT file-system [v4]

[David Beaumont]

> Java changes for supporting preview mode when preview mode resources (with new location flags) are available.
> 
> At the moment, this code will operate on non-preview jimage files (1.0) and act as if no preview resources are available by virtue of the default value for missing attributes and package flags being zero (which matches jimage 1.0).
> 
> This should be reviewed on top of https://github.com/openjdk/valhalla/pull/1618

David Beaumont has updated the pull request incrementally with one additional commit since the last revision:

  Test fixes and feedback changes.
  
  * Renaming slightly confusing "testEncoder" method.
  * Fixing unit tests to use new constructor.

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1619/files
  - new: https://git.openjdk.org/valhalla/pull/1619/files/936b22d1..c9989bdd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1619&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1619&range=02-03

  Stats: 15 lines in 3 files changed: 2 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/valhalla/pull/1619.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1619/head:pull/1619

PR: https://git.openjdk.org/valhalla/pull/1619