Bug 836654 - Process priority fixes

Bug 836654 - Put the expecting-system-message and mozapptype=critical attributes on frames, and hold the "high-priority" wake lock in the phone app.

Combined with the Gecko changes in the bug, this should make is much less likely to kill processes we actually want to keep around.

This is like PR 8053, except it is against gaia/master, which is the right thing to do, aiui.