MM and Crypto wallet breaks web3 sites

Description

As a consequence of #6181, if you install MM and enable crypto wallet on browser, web3 sites like cryptokitties fail to load

Steps to Reproduce

1. Clean profile on 0.69.132
2. Enable Crypto Wallet
3. Install MM from webstore
4. Open a new tab and load cryptokitties.co
5. Open dev tools for the page you can see MM conflicts with another crypto wallet

Actual result:

Sites fail to load due to 2 crypto wallet being enabled. The workaround is to either uninstall MM or disable browser crypto wallet

Expected result:

Should prompt to disable the extension and continue using browser crypto wallet

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	0.69.132 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Linux

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Miscellaneous Information:

cc: @bbondy

[dpazdan]

Expected result: Should prompt to disable the extension and continue using browser crypto wallet

Maybe users can have a choice to pick their default, and/or select on each site (if they want to use multiple solutions).

[dpazdan]

@balresch after disabling Brave wallet, make sure to also close and reopen your browser. If you do not do this step, MetaMask will not function properly. Thanks!