RNGP - Fix RNTester crashing for JSC and safeguard against fetching JSC from Maven Central (#36232)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36232

Seems like RN Tester is crashing for JSC debug/release.

This happens because RN Tester ends up fetching JSC from Maven Central which contains older versions
of the artifacts (r17) which are not compatible with our setup AND are missing `libjsc.so`.

This is happening as our file layout is a bit different than a regular NPM project so
the repository declaration for JSC, being `../jsc-android/dist` from React Native root ends
in the wrong folder.

In this specifically I:
- Add an excludeModule for "org.webkit:android-jsc" on Maven Central inside RNGP
- Remove the allproject{repositories{}} block which was overriding RNGP configuration
- Add a specific repository declaration inside RN Tester to point to where JSC effectively lives

Changelog:
[Internal] [Changed] - RNGP - Fix RNTester crashing for JSC and safeguard against fetching JSC from Maven Central

Reviewed By: sshic

Differential Revision: D43462015

fbshipit-source-id: db830d7567bbf7dd91412df417418aa61a0ca8fe