API Review Feedbacks

1) Rethrow the remoteException instead of returning empty string
 for CarrierConfigManager#getDefaultCarrierServicePackageName()

2) Flip documentation to use “valid” instead of “invalid” for
 ModemActivityInfo.isValid()

Bug: 147804136
Test: Treehugger; Cts
Change-Id: I595837dfb24a09c68e33a732aa24b5b25ff28ac0