only permit text relocations in untrusted_app

The other domains either don't have the same backwards compatibility
issues (isolated_app) or are privileged components that are pretty much
part of the platform and can be expected to meet a higher standard.

It would be possible to expose a build option for disabling the ART JIT,
allowing conditional removal of execmem from some of these domains too
(ones not ever using the WebView, until that's always in isolated_app).

Bug: 20013628
Change-Id: Ic22513157fc8b958b2a3d60381be0c07b5252fa5