Mark GraphicsMapperHidlTest as allowing not being instantiated

Only the variant(s) actually implemented by the device will be
instantiated.

This is consistent with tests for other versioned or optional HALs.

Bug: b/167222309
Test: atest VtsHalGraphicsMapperV2_0TargetTest:GoogleTestVerification#UninstantiatedParameterizedTestSuite<GraphicsMapperHidlTest> -- --abi arm64-v8a
Change-Id: I4f1bfbad47bdbf75578ec48a3a56fb5351e7729a