Fix defaultContentKey regression

This change https://android-review.git.corp.google.com/c/platform/frameworks/support/+/4092597 caused benchmark regression.

Changing implementation from a Pair to a pure String to improve the regression. Some regression is still expected and necessary because the original implementation is insufficient in creating unique contentKeys.

Bug: 542346453
Test: existing tests pass, manual benchmark results
Change-Id: Ieda026fd610f4f0ac5222f3bf61c8087481472b0