Enforce files loaded in DexFile to be read-only

Executables dynamically downloaded can be unexpectedly manipulated,
causing code injection. For apps targeting U+, we'd like to
introduce an enforcement in the platform to make sure files are
marked as read-only.

Test: m
Bug: 218865702
Change-Id: I700af94d47d12514fc345fdc28adf0a1397f6d66
11 files changed