Fix Stack-use-after-scope error in mediametrics_service_fuzzer.cpp

In C2Fuzzer, the lambda variable 'value' was declared on stack and
accessed after the function went out of scope leading to
Stack-use-after-scope error. It is now been declared as a member
of the fuzzer object.

Bug: 192502871
Test: Tested with ASAN:
      Stack-use-after-scope error without the fix and
      no errors reported with fix

Change-Id: I9a23bd6c801092ae64101313f5dc96225fd64f2c