Closes #388: Run Rust FxA calls in single thread context

[carolkng]

Creates an FxaClient-specific single thread context, then moves all launch calls and runBlocking calls (for non-async methods calling into the Rust library) into that context.

r? @csadilek @thomcc

[codecov]

Codecov Report

Merging #399 into master will increase coverage by 0.73%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #399      +/-   ##
============================================
+ Coverage     73.39%   74.12%   +0.73%     
  Complexity      738      738              
============================================
  Files           119      119              
  Lines          2935     2906      -29     
  Branches        411      398      -13     
============================================
  Hits           2154     2154              
+ Misses          568      539      -29     
  Partials        213      213

Impacted Files	Coverage Δ	Complexity Δ
...a/mozilla/components/service/fxa/FirefoxAccount.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...main/java/mozilla/components/service/fxa/Config.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
.../java/mozilla/components/service/fxa/RustObject.kt	0% <0%> (ø)	0 <0> (ø)	⬇️

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[csadilek]

This looks good to me. We have confined all FxA methods to run on a single thread (both the sync and the async ones).

We need to figure out how to release this single thread when we don't need it anymore (to save memory). I've filed a follow-up issue for that: #402

[csadilek]

@pocmo are you ok with this as well? I didn't find an easy way to release the thread on timeout. I filed a follow-up to dig deeper.

[pocmo]

Yes, looks good! 👍

[carolkng]

From Slack discussion with Thom: instead of starting the new thread context and worrying about when to close it, the blocks inside the coroutines can be synchronized on the FxaClient instance.

[csadilek]

@thomcc I think this does it. Want to take another look? We can refactor this later to introduce safeAsynRustCall and safeRustCall methods in RustObject. So, we don't have to do the launch / synchronized dance every time, but I think this is ready to go in now.

[thomcc]

Yeah seems good to me. I'd definitely prefer it if you reduced the duplication (since messing it up == memory unsafety), but if you'd rather do that in a follow up I can deal.

[csadilek]

@carolkng fixed the code duplication by adding the safeAsync and safeSync calls now. Merging, as we have everything we want here ;).