libmediadrm: fix a use of an uninit value

INIT_CHECK() expands to `if (foo) return bar;`, and `EventTimer`'s
destructor uses a value that's only set if `SetAttribute` is called.
This CL flips the INIT_CHECK/EventTimer lines to match getKeyRequest.

Caught by clang's static analyzer:
frameworks/av/include/media/EventMetric.h:155:7: warning: 2nd function
call argument is an uninitialized value
[clang-analyzer-core.CallAndMessage]

Bug: None
Test: TreeHugger
Change-Id: Ie0c4fb8c99a56082e234475e539c2ec4bc8fd948