Implement the PrefsHelper native methods. The previous browser.js
implementation supported "pseudo-prefs" that did not exist as actual
prefs, but was accessible through PrefsHelper. In order to accommodate
these pseudo-prefs, we send observer notifications in order to
communicate with browser.js about prefs that we don't support.
This patch adds specializations of jni::Ref for primitive arrays like
IntArray and FloatArray, so that the arrays can be accessed through the
GetElement, GetElements, and Length members.
When getting nsAppShell from another thread, there could be a race with
nsAppShell being destroyed on the main thread. This patch makes the raw
nsAppShell pointer only accessible from the main thread, and use a
static mutex to coordinate accessing nsAppShell from other threads.
