Prevent NPE in PagedTileLayout

This CL prevents fake drag by checking ViewPager#isFakeDragging, which
is enabled by ViewPager#beginFakeDrag.

This is not a fix of the underlying issue (which is not known) but a
patch to prevent crashing.

Fixes: 149979063
Test: adb shell monkey --throttle 1000 --pct-nav 0 --pct-majornav 0 \
        --pct-touch 30 --ignore-crashes --ignore-timeouts \
        --ignore-security-exceptions -v 2000000
Change-Id: I7da764eaf20fc9d971afe2cb89b232b4efd65a3b