Fixes #3671 BrowserState-SessionStore sync

[keianhzo]

Fixes #3671

Make sure to test the following scenarios:

• New Window creation: from either Tray/Context menu
• New Tab creation: from either the Tabs panel/Context menu
• New Background tab creation (Long press link and open tab from the Context menu)
• Session restore: quit and reopen app with active&suspended sessions (Suspended sessions won't be added to the Browser Store until first activated)
• Session suspension/restore: open 3+ windows in regular model, switch to private, open another 3+ windows and go back and forth between regular/private
• Session recreation: Open several active/suspeded sessions, got to Settings->Privacy and clear cache/data

Logcat filtering by SessionStore will show the BrowserStore-SessionStore sync state including the alive vs suspended totals.

Note: Suspended session will be alive at the SessionStore level but not at the BrowserStore level, we remove them from the BrowserStore when suspended, we only want alive ones there.

[bluemarvin]

If mSessionAdded is in the wrong state, we should fix that instead of passing in boolean to override it.

[MortimerGoro]

not creating the mState in the constructor adds some risks. Parts of the code assume that mState is never null (e..g shutdown, dump listeners, or others)

[keianhzo]

@MortimerGoro Shouldn't happen as we are calling createSession right after creating the Session instance with sets the state but I've added an static method to handle the whole session creation and avoid missing the state creation.