Fix WhenToDreamPickerTest and re-enable.

The test was previously failing because it was passing a mock context
which didn't support all the operations of a real context.

Bug: 216036457
Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.dream.WhenToDreamPickerTest"
Change-Id: I2c769658dc687ef26d184b7ce9065c18ab8b9b8d
1 file changed