Remove support for multiple sdk instances in Firebase App Distribution

[rachaprince]

Remove support for multiple instances of the sdk. Only support the default Firebase app

[google-oss-bot]

Coverage Report

Affected SDKs

• firebase-app-distribution

  SDK overall coverage changed from 66.77% (a336497) to 66.87% (b8cbe492) by +0.10%.

  Filename	Base (a336497)	Head (b8cbe492)	Diff
  FirebaseAppDistribution.java	87.88%	88.96%	+1.08%

Test Logs

• Base (a3364977): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1471578016491507713
• Head (b8cbe492): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3252/check-coverage-changed/1472975670106656780

Notes

HTML coverage reports can be produced locally with ./gradlew <product>:checkCoverage.
Report files are located at <product-build-dir>/reports/jacoco/.

Head commit (b8cbe492) is created by Prow via merging commits: a336497 e5dc742.

[google-oss-bot]

Binary Size Report

Affected SDKs

• firebase-app-distribution

  Type	Base (a336497)	Head (b8cbe492)	Diff
  aar	117 kB	117 kB	-151 B (-0.1%)
  apk (release)	1.55 MB	1.55 MB	-160 B (-0.0%)

• firebase-app-distribution-ktx

  Type	Base (a336497)	Head (b8cbe492)	Diff
  aar	20.0 kB	19.8 kB	-121 B (-0.6%)
  apk (release)	2.03 MB	2.03 MB	-24 B (-0.0%)

Test Logs

• Base (a3364977): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1471578016491507714
• Head (b8cbe492): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3252/binary-size/1472975670106656776

Notes

Head commit (b8cbe492) is created by Prow via merging commits: a336497 e5dc742.

[manny-jimenez]

Seems theres one failing check. Great work!