Clear test rectangle if previous focus is null.

A bug in FolderEditText or DynamicLayout (b/8600683) was causing
the 'rectangle' parameter passed to scrollToRectOrFocus() to be
roughly 1000 feet to the right of the screen.

This bug is normally masked because the focus found in
mLastScrolledFocus never matches the new focus and consequently
the misleading 'rectangle' is nulled. However, on the first
time in to scrollToRectOrFocus when returning to Launcher
from another app, mLastScrolledFocus is null and the code
skips over the place where 'rectangle' is nulled. Without this
clearing it was determined that 'rectangle' was outside the viewable
area and scrolling not required. This is bug 8547155.

This change causes even null values of mLastScrollFocus to be
compared to the new focus thus masking the bug in all situations.
Bug 8600683 will be fixed in a separate CL.

Fixes bug 8547155.

Change-Id: Ic6cb22c42b0e93a9793dd2babc25727c2ba29155