Do not add JCenter dependency when building RN apps

[twometresteve]

Goal

Respond to yesterday's JCenter outage.

Design

The intention of the code in packages/react-native/android/build.gradle was to shield our test fixture builds from JCenter outages, but it also unnecessarily adding a dependency on JCenter for app builds in general.

Changeset

The build script snippet is now moved to only reside within our test fixture build scripts. The get all fixtures to build on CI during the (potentially partial) outage I found it necessary to replace jcenter() with mavenCentral() in a number of locations, as well as removing Flipper from the builds (we don't need it anyway).

Testing

Covered by CI.

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	43.19 kB	13.21 kB
After	43.19 kB	13.21 kB
±	No change	No change

code coverage diff

Coverage values did not change👌.

Total:

Lines	Branches	Functions	Statements
0%(+0%)	0%(+0%)	0%(+0%)	0%(+0%)

Generated by 🚫 dangerJS against d6f9d08

[gingerbenw]

if it's building and passing I'm happy - but review from @lemnik would be appreciated.

[lemnik]

LGTM