Cast packetSize to uint8_t before using.

packetSize is int8_t on AIDL HAL, but packet size may be longer than
128, which will cause a HAL crash.

Bug: 191825295
Test: atest android.media.tv.tuner.cts
Test: atest VtsHalTvTunerTargetTest
Change-Id: I9ee52d3cacab40e69f5f01dbe51c397fa09bfea0