Report P2A messages in json format

[rillian]

Description

Similar to what we've done with P3A, we should start sending P2A reports in json instead of protobuf, so we can can remove the old sending code.

[GeetaSarvadnya]

Verification PASSED on

Brave | 1.41.82 Chromium: 103.0.5060.66 (Official Build) beta (64-bit)
-- | --
Revision | 20b1569438a85e631d15e83eb355e3e326e5da6f-refs/branch-heads/5060@{#1066}
OS | Windows 10 Version 21H2 (Build 19044.1766)

• Verified the test plan from Enable P2A json reporting brave-core#13538
• Confirmed Brave.P2A.* metrics appeared in brave://local-state file
  
• Confirmed Brave.P2A.* reports are sent to the p2a-json.brave.com address, in json format
  
• Confirmed Brave.P2A.* reports are sent to old p2a.brave.com address in encoded binary form
  

[GeetaSarvadnya]

@rillian I could see only one metric in p2a-json.brave.com and p2a.brave.com endpoints today. I was able to see 2 metrics yesterday. Is this because of the issue #23754 or some other reason?. There are only 5 p2a metrics in brave://local-state, I waited for almost an hour, but even then I could see 1 or 2 P2A metrics. Looks like P2A metrics are also affected by the issue #23754. Instead of failing two issues, I am adding the PASS label to this issue but I will add comments in the logged issue to verify the P2A metrics as well once the PR is up.

[rillian]

@GeetaSarvadnya I think I understand the cause of #23754 now, and it would affect p2a and p3a messages equally, so I expect the missing p2a reports have the same cause.

[GeetaSarvadnya]

All the p2a metrics are sent to the end point p2a-json.brave.com and p2a.brave.com, verified this in the issue #23147 (comment)

[Uni-verse]

Verification completed on Samsung Galaxy S21 & Galaxy Tab S7 using

Brave	1.41.94 Chromium: 103.0.5060.114 (Official Build) (64-bit) 
Revision	a1c2360c5b02a6d4d6ab33796ad8a268a6128226-refs/branch-heads/5060@{#1124}
OS	Android 12; Build/SP1A.210812.016  

encountered - #23922

Created above follow up issue to fix the missing p2a metrics on android.