Call isEmpty() instead of constructing a new object to compare

A recent change in Skia
(https://skia-review.googlesource.com/c/skia/+/573636) moves operator==
into the cpp file, where this file cannot access it due to libhwui's map
file. Call isEmpty, which is still in the header file, and skips
unnecessary creation of a new object anyway.

Bug: NA
Test: AImageDecoderTest
Change-Id: I1dec89a38e248138ace364e8cb7259eb777e8871