Make HEIF decoding tests optional for devices without HEVC support.

CDD has been updated to explicitly state HEIC/HEIF support
is a MUST only when the device supports HEVC decoder:
https://android-review.googlesource.com/1469663/.

This CL revives the condition in the CTS test code.

Bug: 168422730
Test: CtsGraphicsTestCases
Change-Id: I40bedacff37dbb9373aafce26ebff5dee2c64fca
(cherry picked from commit 6b70af637c2334759277c9fc6306e5b6c59f3c15)