Disable transcoding by default if no HEVC support

This patch by default disables transcoding if HEVC decoder is not
supported. It may still be enabled/disabled via persistet properties
and/or device_config settings.

Bug: 270324498
Bug: 270320827
Test: run cts -m CtsMediaProviderTranscodeTests
Test: run cts -m CtsMediaTranscodingTestCases
Change-Id: I63ac7f4dd5103bea3f67a491a3fe030bafce075c
Signed-off-by: Slawomir Rosek <srosek@google.com>