Don't crash when passing null to requestChildFocus

Add null checks to ScrollView and HorizontalScrollView for checking
the revealOnFocusHint. This should never happen in code called by
the framework, but some apps were hitting it.

Bug: 36379645
Test: none
Change-Id: I220eb88d82126ff08f47a7c2a7fbdddebf07de81
2 files changed