Fix crash when clicking item in Battery screen

"last full charge/Screen usage since full charge" goes crash when
clicking on it. This is because The default behaviour for
PowerGaugePreference is to start detail page and the above two items
are PowerGaugePreference.

This cl set these two items as non-selectable.

Bug: 37080048
Test: make -j40 SettingsTest
Change-Id: Ide4e151dcd0b44fb795a5753c91f2197b2a24a56