Rewards panel loads slowly state when network is slow

[zenparsing]

Because some calls to the chrome.braveRewards API require network access, when the network is unavailable or slow the Rewards panel can stay in the "loading" state for some time. We should be able to display the current state to the user and update the UI when the data becomes available.

[zenparsing]

See #19287 for a more general issue.

[GeetaSarvadnya]

Verification PASSED on

Brave | 1.40.75 Chromium: 102.0.5005.61 (Official Build) beta (64-bit)
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Revision | 0e59bcc00cc4985ce39ad31c150065f159d95ad3-refs/branch-heads/5005@{#819}
OS | Windows 10 Version 21H2 (Build 19044.1706)

• Verified the test plan from Show Rewards panel content if Rewards API is unresponsive brave-core#13329
• Confirmed that clicking on the BR panel display content within 3 seconds when the network is disconnected

[stephendonner]

Verified PASSED using

Brave	1.40.82 Chromium: 102.0.5005.78 (Official Build) beta (x86_64)
Revision	df6dbb5a9fd82af3f567198af2eb5fb4876ef99c-refs/branch-heads/5005_59@{#3}
OS	macOS Version 11.6.6 (Build 20G624)

Followed the testplan from brave/brave-core#13329

Confirmed that clicking on the Brave Rewards panel displays content within 3 seconds when the network is disconnected

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First, tried to reproduce the issue using Rewards staging env:

Brave	1.39.111 Chromium: 102.0.5005.61 (Official Build) (x86_64)
Revision	0e59bcc00cc4985ce39ad31c150065f159d95ad3-refs/branch-heads/5005@{#819}
OS	macOS Version 11.6.6 (Build 20G624)

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Verification passed on

Brave	1.40.87 Chromium: 102.0.5005.78 (Official Build) beta (64-bit)
Revision	df6dbb5a9fd82af3f567198af2eb5fb4876ef99c-refs/branch-heads/5005_59@{#3}
OS	Ubuntu 18.04 LTS

Verified the testplan from brave/brave-core#13329

Confirmed that clicking on the Brave Rewards panel displays content within 3 seconds when the network is disconnected