Fix focus behavior of ValidatedEditTextPreference

The bug report was that the cursor is set to the beginning of the
EditText for wifi hotspot password field, instead of at the end (or
having the entire field selected). This fix makes it so that all
ValidatedEditTextPreference's will put the cursor at the end of the
EditText.

Bug: 65413206
Test: make RunSettingsRoboTests
Change-Id: I23f3bb1b3f1b49b2f193c0ae2e103eae5c1a1019