Implement getStorageInfo.

This retrieves storage info for a certain package and database.

Bug: 182909475
Test: atest -m -c --rebuild-module-info CtsAppSearchTestCases
FrameworksCoreTests: android.app.appsearch
FrameworksServicesTests: com.android.server.appsearch
Change-Id: I524e0b2ada919981f50722f01ecd3f62b2ae4f43