Check for a specific flutter asset file instead of listing the path

[mrober]

Check for a specific flutter asset file instead of listing the path.

This should fix issue #3605 caused by the fact that listing an asset path may be slow in some cases.

[google-oss-bot]

Coverage Report ¹

Affected Products

No changes between base commit (26c905b) and merge commit (c086eb0).

Test Logs

• Base (26c905b): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1512215363645870080
• Merge (c086eb0): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3631/check-coverage-changed/1512456421424238596

Notes

• Commit (c086eb0) is created by Prow via merging PR base commit (26c905b) and head commit (2926d63).
• Run gradle <product>:checkCoverage to produce HTML coverage reports locally. After gradle commands finished, report files can be found under <product-build-dir>/reports/jacoco/.

1. https://storage.googleapis.com/firebase-sdk-metric-reports/14iqhRYuog.html

[google-oss-bot]

Size Report ¹

Affected Products

• firebase-crashlytics

  Type	Base (26c905b)	Merge (c086eb0)	Diff
  aar	340 kB	340 kB	+156 B (+0.0%)
  apk (aggressive)	215 kB	216 kB	+72 B (+0.0%)
  apk (release)	891 kB	891 kB	-36 B (-0.0%)

Test Logs

• Base (26c905b): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1512215363645870081
• Merge (c086eb0): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3631/binary-size/1512456421424238592

Notes

• Commit (c086eb0) is created by Prow via merging PR base commit (26c905b) and head commit (2926d63).

1. https://storage.googleapis.com/firebase-sdk-metric-reports/3PpYlULdbF.html

[argzdev]

I think this is a good fix, since AssetManager.list() have been reported to have slow performance in a few cases. Hopefully checking the file directly will address the issue for Android 6.0 devices.