fix(app, android): minSdk should be 19 to match firebase-android-sdk

[SaeedZhiany]

Description

Related issues

As @mikehardy requested in this comment I have tested react-native-firebase on projects with minSdk 19 and it works fine for me.

Release Summary

Downgraded default minSdk from 21 to 19 to support more android devices.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

It worth mentioning that I'm currently using the cloud messaging package in my project. if anyone is willing to test this PR on the other packages, feel free to clean its project and then apply the same simple change on this PR, and test their packages' functionality, and report the result on this Thread.

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[codecov]

Codecov Report

Merging #5984 (4970301) into main (432a7bd) will increase coverage by 27.51%.
The diff coverage is n/a.

❗ Current head 4970301 differs from pull request most recent head c1f38ea. Consider uploading reports for the commit c1f38ea to get more accurate results

@@              Coverage Diff              @@
##               main    #5984       +/-   ##
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+ Coverage     25.45%   52.95%   +27.51%     
- Complexity        0      622      +622     
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  Files            97      208      +111     
  Lines          4245    10189     +5944     
  Branches       1033     1619      +586     
=============================================
+ Hits           1080     5395     +4315     
- Misses         2571     4540     +1969     
+ Partials        594      254      -340     

[mikehardy]

Nice! Thank you. I think for this one it simply wouldn't build if it was not going to work, and this matches the documented min sdk of firebase-android-sdk so should be fine