Use CLOCK_BOOTTIME for default sensor implementation

The sensor implementation used to generate periodic signals according to
CLOCK_REALTIME. The clock can be adjusted by user or synchronization
service and results in wrong period. This commit changes it to
CLOCK_BOOTTIME which is monotonic.

Test: atest VtsHalSensorsV2_0TargetTest
Test: atest VtsHalSensorsV2_1TargetTest
Test: atest CtsSensorTestCases
Bug: 194030777
Change-Id: Iac39c93d6d2518b1abe8fc07fd695b9bb2cf30a5