Fix UnsupportedOperationException in Display Settings

Remove android:textColorHint from Theme.Settings to fix the regression.

This is a quick fix, each EditText should set its hintTextColor
attribute before there is a fix to apply the attribute globally in
Settings APP.

Bug: 188405076
Test: manually launch 'Font size' and 'Display size'.
Change-Id: If3313be47e0c3e8ce9d41a6e87cdf2e7067ce54b