Implementation changes:
- browser_specific_settings.gecko: treat unrecognized keys as warnings
instead of hard errors.
- browser_specific_settings.gecko_android: treat unrecognized keys as
warnings instead of silently ignoring them.
Test change (besides adding coverage for the implementation changes):
- Moved test_non_gecko_bss_install from test_webextension_install.js to
test_ext_manifest.js, because the test is related to manifest parsing
and the test is currently skipped on Android.
Differential Revision: https://phabricator.services.mozilla.com/D199798
e8191ab1c6