check-flagged-apis: allow / chars in Symbol names

Allow forward slash characters (/) in Symbol names: when adding support
for method arguments, this will be needed.

The current implementation does not change; forward slash conversions to
dots still happen, but now explicitly at the call site of Symbol.create.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: Ia860d7b0c8703fcc56fec6ea722cf995ccf20cd0