Add some TestAPIs for replacing some @hide API usages in ExtServices's test

Added a builder class for UsageStats and StorageVolume. This allows
us to create these data classes for testing.

Currently, a test in ExtServices creates these objects by using some
hidden APIs. Those tests are part of the mainline test suite, we should
not use hiden APIs there. Otherwise, the test APK cannot be installed on
older platforms.

Test: atest ExtservicesUnitTest

Bug: 182546925
Change-Id: I32939dedfa55233dfb27337cee8818a9dc25d786