1) TestStorage now supports passing in a custom ContentResolver other than the the content resolver of the app under test in Instrumentation.

[copybara-androidxtest]

1. TestStorage now supports passing in a custom ContentResolver other than the the content resolver of the app under test in Instrumentation.
2. Uses the AndroidX Experimental annotation for the TestStorage APIs;
3. Adds comments to explain the pathname argument, and includes naming conventions for argument names.

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