Don't retain a bitmap unnecessarily

Due to a limitation in the GC, LazyTaskWriterThread would always retain
the last item it processed.  Extract the second half into a separate
method to clarify the scope of the variable 'item' to the GC.

Bug: 64438652
Test: Launch several apps and check with ahat
Change-Id: Ie7357927ae61a8731285d14743187a2d811fbf8c