system/stack/a2dp: Prevent unsigned overflows in timestamp computation

The timestamp can overflow the uint32_t variable that stores it,
given a long enough stream (25h @ 48KHz). UBSAN is always enabled
and crashes the stack when that does happen.

Bug: 337056284
Test: m com.android.btservices
Test: Manual |
   - Disable A2DP Offload in developer options |
   - Connect A2DP Sink |
   - Start A2DP stream any codec in aptX, aptX HD, AAC, LDAC, Opus |
   - Validate stream output
Flag: EXEMPT no logical change
Change-Id: I2f0ecc47fd3287409197bb8e1d39de5974733116