fix(#WindowMagnification): adjust Settings getter def to true for key...

fix(#WindowMagnification): adjust Settings getter def to true for key ACCESSIBILITY_ALLOW_DIAGONAL_SCROLLING

As b/279397954, the allow diagonal scrolling default should be true by default. Therefore, we adjust the Settings getter to return true if Settings.Secure.ACCESSIBILITY_ALLOW_DIAGONAL_SCROLLING value is UN_ASSIGNED.

Bug: 279397954
Test: manually
      atest WindowMagnificationControllerTest
      atest WindowMagnificationSettingsTest
Change-Id: I244903a3fbf11a73998f440941b82c57f8ac4ef3