Fix integer overflow in GKI timer calculation

Fix an integer overflow in the GKI timer elapsed time calculation that
would cause Bluetooth Stereo Audio (A2DP) playback to stop suddenly
after a period of INT32_MAX microseconds (about 35 minutes) of
continuous uptime of the phone/tablet.

Bug: 16412377
Change-Id: I28cd3155a23444e41b26b3f3b8424cc2a3c5bf91