b2g profile needs to be moved outside of extension install root

[kmaglione]

The simulator currently runs b2g using a profile directory stored directly under its install root. There are several problems with this:

1. Changing the modification time of an extension's install root has been known to cause problems with the add-on manager in the past.

2. Any time a file within an extension install root is modified, the startup cache is flushed, and every time the simulator runs, files are modified.

3. At the moment, the add-on manager stats every file in an unpacked extension at startup. Given the number of files in a b2g profile, it has a significant performance cost.

We also have a policy on AMO that forbids add-ons from modifying files within their install root.

[mykmelez]

cc: @ochameau