[foreign-memaccess+abi] RFR: Fix visibility issues

[Per Minborg]

This PR proposes to fix two visibility issues where package private interfaces are exposed via another class' public metods.

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

Commit messages:
 - Fix visibility issues

Changes: https://git.openjdk.org/panama-foreign/pull/891/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=891&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/891.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/891/head:pull/891

PR: https://git.openjdk.org/panama-foreign/pull/891