Fix abort when DeleteWeakGlobalRef in ~FrontendClientCallbackImpl()

This week global ref was newed once bug deleted twice. It triggered an
abort on userdebug build when checkJni is enabled.

Bug: 195591944
Fix: 195591944
Test: atest android.media.tv.tuner.cts
Change-Id: I0b46628a0fc192708201c3c8b536b7ad1a4ea8f1