Workaround an interpreter behavior to fix leak

When a local variable (cachedThumbnailData) gets out of scope,
the interpreter might still keep a reference to it in the
register. Thus, we extract the handling of this local variable
into a method such that it gets removed from the register.

Test: Open recents, go home, take dump, make sure no reference
from HandlerThread to ThumbnailData is still existing.
Fixes: 37630958

Change-Id: If25456b212341be7a66882a0adfd5afb4a64ce23