[Desktop] Migrate uphold_anon_address into external_wallets.uphold

[NejcZdovc]

Right now uphold_anon_address lives in brave.rewards and we don't clear it when we disconnecting wallet. This should be migrated into brave.rewards.external_wallets.uphold, so that when we disconnect wallet we remove everything.

When migration is done we should clear and deprecate kStateUpholdAnonAddress value.

[LaurenWags]

Verified passed with

Brave	1.13.76 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
Revision	4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS	macOS Version 10.14.6 (Build 18G3020)

Plan 1 from brave/brave-core#6063
Prior to connecting KYC'd uphold wallet, this is what Preferences file looked like:

After connecting KYC'd uphold wallet, this is what Preferences file looked like:

Plan 2 from brave/brave-core#6063
Using 1.12.x, this is what Preferences file looked like with KYC'd wallet:

Upgrading to 1.13.x, this is what the Preferences file looked like:

---

Verification passed on

Brave | 1.13.76 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
-- | --
Revision | 4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS | Windows 10 OS Version 1903 (Build 18362.1016)

Plan 1 from brave/brave-core#6063
Prior to connecting KYC'd uphold wallet, this is what Preferences file looked like:

After connecting KYC'd uphold wallet, this is what Preferences file looked like:

Plan 2 from brave/brave-core#6063
Using 1.12.x, this is what Preferences file looked like with KYC'd wallet:

Upgrading to 1.13.x, this is what the Preferences file looked like:

---

Verification passed on

Brave	1.13.77 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
Revision	4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS	Ubuntu 18.04 LTS

Plan 1 from brave/brave-core#6063

After connecting KYC'd uphold wallet, this is what Preferences file looked like:

Plan 2 from brave/brave-core#6063
Using 1.12.x, this is what Preferences file looked like with KYC'd wallet:

Upgrading to 1.13.x, this is what the Preferences file looked like: