Updated the initialization of upgrader

[larryaasen]

• Updated the initialization of upgrader to use a future that will allow the initialize method to be called multiple times. This does not require any changes by app code.
• Added new example using upgrader inside a stateless widget.
• Added additional try/catch logic to report App Store API errors.
• Fixed unit tests that were broken due to the initialization changes.

[codecov[bot]]

Codecov Report

Base: 92.87% // Head: 93.06% // Increases project coverage by +0.18% :tada:

Coverage data is based on head (5a7586e) compared to base (dc38343). Patch coverage: 78.04% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #255 +/- ## ========================================== + Coverage 92.87% 93.06% +0.18% ========================================== Files 10 10 Lines 969 980 +11 ========================================== + Hits 900 912 +12 + Misses 69 68 -1 ``` | [Impacted Files](https://codecov.io/gh/larryaasen/upgrader/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen) | Coverage Δ | | |---|---|---| | [lib/src/itunes\_search\_api.dart](https://codecov.io/gh/larryaasen/upgrader/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen#diff-bGliL3NyYy9pdHVuZXNfc2VhcmNoX2FwaS5kYXJ0) | `80.28% <66.66%> (+1.71%)` | :arrow_up: | | [lib/src/upgrader.dart](https://codecov.io/gh/larryaasen/upgrader/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen#diff-bGliL3NyYy91cGdyYWRlci5kYXJ0) | `90.17% <78.94%> (-0.01%)` | :arrow_down: | | [lib/src/play\_store\_search\_api.dart](https://codecov.io/gh/larryaasen/upgrader/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen#diff-bGliL3NyYy9wbGF5X3N0b3JlX3NlYXJjaF9hcGkuZGFydA==) | `95.95% <0.00%> (+1.01%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen)

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