Deprecate android:isSplitRequired

Since android:requiredSplitTypes and android:splitTypes are introduced
android:isSplitRequired is been a bit too ambiguous to be very useful
to this point. requiredSplitTypes provides a more specific form of
isSplitRequired by defining exactly what is required and when provided
as anything other than empty string implies isSplitRequired="true".

Bug: 198708193
Test: build
Change-Id: Ic3803794df80fdbfc12c07ae84f3d46e2cf74db6