updateAutofillValue() was crashing some apps when the mText was not set at the
time it was called. One solution would be to not set mText at all - since the
Autofill Service should rely only on getAutofillValue() - but that could break
existing services.
Hence, a safer solution is to set that field if it's null.
Test: existing CtsAutoFillServiceTestCases tests pass
Test: manual verification using Fly Delta app
Fixes: 62751039
Change-Id: I91a8e0ed5db4148f5eb5729b8e254aa3531f15e4
266752317b