check-flagged-apis: add Symbol value class

check-flagged-apis will read contents from various sources, which use
different formats to represent the same piece information (e.g.
"class#field" or "<class><field>").

Introduce a Symbol value class to represent any API (i.e. a class, field
or method) in a unified format.

Bug: 334870672
Test: m check-flagged-apis && check-flagged-apis
Change-Id: Id9404294a87b23a9d43e5e13ce39ea5a92608e33