Fixed updateAutofillValue() when mText is not set.

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