Change FATAL message to WARNING message in FindClass

When unable to find the specified class, FindClass was logging a
FATAL message. This terminated the program instead of simply returning
a null pointer as expected. The FATAL message has been replaced with
a WARNING message to fix this behavior.

Bug: 112338190
Test: m -j
Change-Id: I91cc0747a45bff72aa0ffa8f0e1c8fa238469ddf
(cherry picked from commit 7cac35ff)
Merged-In: I91cc0747a45bff72aa0ffa8f0e1c8fa238469ddf