refactor: Add background Redux slice as proxy for unflattened background state

[MajorLift]

Description

Related issues

Fixes:

Manual testing steps

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Screenshots/Recordings

Before

After

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[metamaskbot]

Builds ready [2f24443]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
  • content-script: 0
  • offscreen: 0

Page Load Metrics (1676 ± 43 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	501	1871	1624	272	131
		domContentLoaded	1484	1857	1656	85	41
		load	1493	1874	1676	89	43
		domInteractive	24	93	38	17	8
		backgroundConnect	10	57	28	16	8
		firstReactRender	15	72	34	24	11
		getState	5	56	12	14	7
		initialActions	0	0	0	0	0
		loadScripts	1096	1389	1235	74	35
		setupStore	8	68	19	19	9
		uiStartup	1723	2168	1917	106	51

[metamaskbot]

Builds ready [6d501d9]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
  • content-script: 0
  • offscreen: 0

Page Load Metrics (1928 ± 144 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	1600	2725	1932	299	144
		domContentLoaded	1578	2680	1881	281	135
		load	1590	2728	1928	300	144
		domInteractive	27	116	56	33	16
		backgroundConnect	12	172	50	43	21
		firstReactRender	15	108	41	25	12
		getState	6	79	24	20	10
		initialActions	0	1	0	0	0
		loadScripts	1205	2028	1428	223	107
		setupStore	7	94	23	25	12
		uiStartup	1767	3774	2228	455	218

[metamaskbot]

Builds ready [b97221a]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
  • content-script: 0
  • offscreen: 0

Page Load Metrics (1606 ± 51 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	330	1916	1487	381	183
		domContentLoaded	1412	1836	1588	107	52
		load	1423	1847	1606	107	51
		domInteractive	24	88	31	14	7
		backgroundConnect	8	53	22	15	7
		firstReactRender	14	82	43	26	12
		getState	5	58	9	11	5
		initialActions	0	1	0	0	0
		loadScripts	1019	1434	1172	91	44
		setupStore	7	53	14	13	6
		uiStartup	1660	2133	1840	115	55