fix: Fenix does not require WRITE_EXTERNAL_STORAGE permission to be granted

[rpl]

Fixes #2304

For Fenix we don't need the WRITE_EXTERNAL_STORAGE permission to be granted:

• it was required for Fennec because web-ext run was also running it in a separate temporary profile as we do on desktop
• but Fennec doesn't support that and so the WRITE_EXTERNAL_STORAGE permission isn't really necessary
• We still need the READ_EXTERNAL_STORAGE permission to have been granted, to allow Fennec to load the xpi file that web-ext run uploads to the android emulator or device

TODO:

• confirm with Rob that we are both ok with the strategy drafted in this PR (or agree on an alternative one)
• add a more specific test case for the behavior expected
• test on an android 11 emulator or device and a recent Fenix and GeckoViewExample builds

[rpl]

@Rob--W this is currently mainly a draft of the kind of fix we may use and then even release on npm in a minor version:

the idea is to only make the check for the read permission still mandatory, and instead only run a "non blocking" check for the write permission (which just log a debug message, to avoid confusing the user with a warning message that is unlikely to apply to most users nowdays, but still allow us to get it in the verbose logs if we are investigating an issue reported to us).

Let me know wdyt.

[codecov]

Codecov Report

Merging #2306 (e828938) into master (c884696) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2306   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files          32       32           
  Lines        1699     1699           
=======================================
  Hits         1697     1697           
  Misses          2        2           

Impacted Files	Coverage Δ
src/extension-runners/firefox-android.js	100.00% <ø> (ø)

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[Rob--W]

I just tested, and the read permission isn't even needed to run an extension in Fenix. We can delete lots of code.

[rpl]

@Rob--W have you checked that the permission wasn't actually already granted?
some more details about what STR you actually tested manually would be helpful to assess if that was enough to ensure we are not missing some legit corner case.

[rpl]

@Rob--W also, I would like to release this into a minor or patch version, and so "less changes" would make me feel way less concerned about releasing it in a "non major" new release (than "remove a lot of code").

I would be obviously more than happy about "remove a lot of code" in a separate follow up.

[Rob--W]

Filed #2312