Change mCarrierServiceUid from int[] to SparseIntArray

Because followup changes will stop using synchronous getters
for getting the service package UIDs, it will be a lot more
convenient if this array is automatically sized.

Test: FrameworksNetTests
Bug: 236669534
Change-Id: I2df8442727b5f17bd2d4e55a8795dc9210beb74b