Add development_platform for Unity crash reports

[mrichards]

Crashlytics reports sent via DataTransport erroneously did not set the
development_platform field for Unity apps. This results in incorrect
internal metrics for Unity apps. b/171214452

[mrichards]

@mrwillis21 I'm looking for feedback on the overall approach, and if you think it is worth adding in the install_source field in a similar way.

[google-oss-bot]

Coverage Report

Affected SDKs

No changes between base commit (51c995e) and head commit (3887d9f8).

Test Logs

• Base (51c995ec): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1321147565336432644
• Head (3887d9f8): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/2108/check-coverage-changed/1321179904485625857

Notes

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

Head commit (3887d9f8) is created by Prow via merging commits: 51c995e f9e632a.

[google-oss-bot]

Binary Size Report

Affected SDKs

• firebase-crashlytics

  Type	Base (51c995e)	Head (3887d9f8)	Diff
  aar	411 kB	412 kB	+1.05 kB (+0.3%)
  apk (aggressive)	326 kB	326 kB	+296 B (+0.1%)
  apk (release)	1.04 MB	1.04 MB	+292 B (+0.0%)

• firebase-installations

  Type	Base (51c995e)	Head (3887d9f8)	Diff
  aar	62.0 kB	62.0 kB	-4 B (-0.0%)

Test Logs

• Base (51c995ec): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1321147565336432642
• Head (3887d9f8): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/2108/binary-size/1321179904485625856

Notes

Head commit (3887d9f8) is created by Prow via merging commits: 51c995e f9e632a.

[mrwillis21]

One minor nit, but otherwise this seems great to me.

[mrichards]

/retest

[google-oss-bot]

@mrichards: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
smoke-tests	f9e632a	link	/test smoke-tests

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