More adb cleanup.

This removes adb_dirstart and adb_dirstop. It also fixes a couple of memory
leaks by switching to std::string. This also fixes the bug in the previous
change --- mkdirs is given input like "/system/bin/sh" and only expected to
create "/system/bin". In a later change, we should remove mkdirs and only
expose the intended "unlink && mkdirs && create" functionality.

Change-Id: I30289dc1b3dff575cc1b158d993652178f587552