RTDB Optimization: Set initial capacity of the string builder

[jsdt]

This is a minor optimization to reduce some allocations in the RTDB client.

[google-oss-bot]

Coverage Report ¹

Affected Products

• firebase-database

  Overall coverage changed from 50.21% (889bf9a) to 50.16% (38b4c2c) by -0.05%.

  Filename	Base (889bf9a)	Merge (38b4c2c)	Diff
  BooleanNode.java	92.31%	100.00%	+7.69%
  ChildChangeAccumulator.java	96.67%	83.33%	-13.33%

Test Logs

• Base (889bf9a): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1522266504354074629
• Merge (38b4c2c): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3701/check-coverage-changed/1522344644443115520

Notes

• Commit (38b4c2c) is created by Prow via merging PR base commit (889bf9a) and head commit (27ca577).
• 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/TqWscbyprS.html

[google-oss-bot]

Size Report ¹

Affected Products

• firebase-database

  Type	Base (889bf9a)	Merge (38b4c2c)	Diff
  aar	488 kB	488 kB	+63 B (+0.0%)
  apk (release)	1.14 MB	1.14 MB	+96 B (+0.0%)

Test Logs

• Base (889bf9a): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1522266504354074626
• Merge (38b4c2c): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3701/binary-size/1522344644430532609

Notes

• Commit (38b4c2c) is created by Prow via merging PR base commit (889bf9a) and head commit (27ca577).

1. https://storage.googleapis.com/firebase-sdk-metric-reports/A2a8nf78lt.html