improve handling of last selected provider

[brad-decker]

In #10084 I addressed an issue where the previous provider was not being set when switching to a custom RPC. In this PR, I move the logic for keeping track of the previously connected network to the network controller. Removing the need to keep track of this in appState, and exposing a method to simply rollback to the previous provider.

[metamaskbot]

Builds ready [2a88627]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• code coverage: Report
• dep viz: background
• all artifacts

Page Load Metrics (507 ± 22 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	31	72	47	9	4
		domContentLoaded	325	555	505	46	22
		load	327	556	507	46	22
		domInteractive	325	555	505	46	22

[Gudahtt]

LGTM! This looks much simpler than before