Remove FieldMask

[schmidt-sebastian]

This PR:

• Removes FieldMask
• Re-uses testing logic for verifyRoundTrip
• Uncovers versioning issue in overlay

[google-oss-bot]

Coverage Report

Affected SDKs

No changes between base commit (35d1595) and head commit (8e8f3d2a).

Test Logs

• Base (35d15957): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1438519982236372993
• Head (8e8f3d2a): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/2983/check-coverage-changed/1438965500641021956

Notes

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

Head commit (8e8f3d2a6) is created by Prow via merging commits: 35d1595 618d559.

[google-oss-bot]

Binary Size Report

Affected SDKs

• firebase-firestore

  Type	Base (35d1595)	Head (8e8f3d2a)	Diff
  aar	1.19 MB	1.19 MB	-713 B (-0.1%)
  apk (release)	3.24 MB	3.24 MB	-184 B (-0.0%)

Test Logs

• Base (35d15957): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1438519982236372992
• Head (8e8f3d2a): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/2983/binary-size/1438965500641021957

Notes

Head commit (8e8f3d2a6) is created by Prow via merging commits: 35d1595 618d559.

[google-oss-bot]

@schmidt-sebastian: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
check-changed	618d559	link	/test check-changed

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