Move BufferHubMetadata off pdx::fileHandle

Use android::base::unique_fd to replace LocalHandle. For BorrowedHandle,
a const reference of the unique_fd is returned.

Updated the test cases to fit in current behavior.

Test: BufferHubMetadata_test, BufferHubBuffer_test, BufferNode_test,
buffer_hub-test (passed)
Fix: 118888072

Change-Id: I34de335ed9a10864ac226cd4d7d261ba0078045d