Designated balance call

[NejcZdovc]

Currently we are getting balance based on wallet properties that we are getting them from the server. With Gemini we need to handle multiple wallets meaning that we need to have designated call to get balance which will combine wallet properties with other wallet balances. In this phase we should just prepare this call with returning balance from wallet properties only.

We need to remove balance and rates from wallet properties so that people would not use old API anymore.

[GeetaSarvadnya]

Verification passed on

Brave	0.68.113 Chromium: 76.0.3809.62 (Official Build) beta (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified the test plan from Adds new balance call brave-core#2715 (Staging env)

Verification passed on

Brave	0.68.113 Chromium: 76.0.3809.62 (Official Build) beta (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Ubuntu 18.04 LTS

• Verified the test plan from Adds new balance call brave-core#2715 (Staging env)

Verified passed with

Brave	0.68.119 Chromium: 76.0.3809.87 (Official Build) beta (64-bit)
Revision	111fe1e15d5ced26080a7dc239bcfe70f6c49aad-refs/branch-heads/3809@{#967}
OS	Mac OS X

• Verified the test plan from Adds new balance call brave-core#2715 (Staging env)