Implement the deletion request ping in Glean

[badboy]

https://bugzilla.mozilla.org/show_bug.cgi?id=1587095

[travis79]

A question about enabling cleartext and a couple nits about setting initial state of preference toggles in sample applications.

[Dexterp37]

Thanks for working on this. This looks good as a start, I have no major complain, just a bunch of nits here and there on the Android part. I'm leaving the iOS part to @travis79 .

[travis79]

I'm good, with @Dexterp37's concerns addressed

[badboy]

Blocked by #528

[codecov-io]

Codecov Report

Merging #526 into master will decrease coverage by 0.32%.
The diff coverage is 46.73%.

@@             Coverage Diff             @@
##             master    #526      +/-   ##
===========================================
- Coverage     75.92%   75.6%   -0.33%     
- Complexity      333     334       +1     
===========================================
  Files            96      98       +2     
  Lines          5649    5730      +81     
  Branches        703     714      +11     
===========================================
+ Hits           4289    4332      +43     
- Misses          842     879      +37     
- Partials        518     519       +1

Impacted Files	Coverage Δ	Complexity Δ
glean-core/ios/Glean/Net/HttpPingUploader.swift	82.65% <100%> (+0.17%)	0 <0> (ø)	⬇️
glean-core/ios/Glean/Glean.swift	92.63% <100%> (+0.28%)	0 <0> (ø)	⬇️
...oid/src/main/java/mozilla/telemetry/glean/Glean.kt	88.82% <100%> (+0.05%)	1 <0> (ø)	⬇️
...emetry/glean/scheduler/DeletionPingUploadWorker.kt	25% <25%> (ø)	1 <1> (?)
glean-core/src/lib.rs	62.43% <50%> (-0.67%)	0 <0> (ø)
glean-core/src/internal_pings.rs	58.33% <58.33%> (ø)	0 <0> (?)
glean-core/src/ping/mod.rs	68.23% <71.42%> (+0.57%)	0 <0> (ø)	⬇️
glean-core/src/metrics/datetime.rs	44.23% <0%> (+3.84%)	0% <0%> (ø)	⬇️
... and 1 more

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