Add indicator that an elf is memory backed.

Modify the unwinder library to indicate that at least one of the stack
frames contains an elf file that is unreadable.

Modify debuggerd to display a note about the unreadable frame and a possible
way to fix it.

Bug: 129769339

Test: New unit tests pass.
Test: Ran an app that crashes and has an unreadable file and verified the
Test: message is displayed. Then setenforce 0 and verify the message is
Test: not displayed.
Change-Id: Ibc4fe1d117e9b5840290454e90914ddc698d3cc2