Switch MemoryRemote to use ptrace.

Some processes will fail to read when using process_vm_read, so switch
back to ptrace for now. For example, the system_server process only
gets two frames without this fix.

Fix up some of the remote memory read unit tests to allow passing
even when using ptrace.

Bug: 70160908

Test: All unit tests pass, debuggerd -b <system_server pid> works.
Change-Id: I37c67bf9c480d7268c9fe32dad6a8d78a4020b85