Add runas_app domain to allow running app data file via run-as.

Calling execve() on files in an app's home directory isn't allowed
for targetApi >=29. But this is needed by simpleperf to profile
a debuggable app via run-as.
So workaround it by adding runas_app domain, which allows running
app data file. And add a rule in seapp_contexts to use runas_app
domain for setcontext requests from run-as.

Bug: 118737210
Test: boot marlin and run CtsSimpleperfTestCases.
Change-Id: I5c3b54c95337d6d8192861757b858708174ebfd5