Set default shields mode depending on whether it is a new or existing Brave user

[davidtemkin]

Test plan

See brave/brave-core#3154

Description

In recent versions of Brave, there is a new "simple" shields view, as well an "advanced" view. The "advanced" view is very similar to the pre-existing single-view version.

In light of that:

• Existing Brave users should receive the advanced view by default.

• New Brave users should see the simple view by default (and with one click, they can switch to the advanced view, for all sites).

(IMPORTANT NOTE: The two-mode shields implementation is already in Dev. This particular fix should be uplifted so that by the time the two-mode shields panel hits release, or even Beta, we have already made this change, to keep things familiar for existing users.)

[LaurenWags]

Verified passed with

Brave	0.69.124 Chromium: 76.0.3809.132 (Official Build) (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Mac OS X

• Verified test plan from Default shield advanced view to TRUE for existing users. brave-core#3154
• Encountered and logged existing users getting warning message when opening shields for the first time - follow up to 5582 #5924

Verification passed on

Brave	0.69.124 Chromium: 76.0.3809.132 (Official Build) (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified test plan from Default shield advanced view to TRUE for existing users. brave-core#3154
• Encountered existing users getting warning message when opening shields for the first time - follow up to 5582 #5924

Verification passed on

Brave	0.69.124 Chromium: 76.0.3809.132 (Official Build) (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Ubuntu 18.04 LTS

• Verified test plan from Default shield advanced view to TRUE for existing users. brave-core#3154
• Encountered existing users getting warning message when opening shields for the first time - follow up to 5582 #5924