Change clock used for wakelock tracing

Perfetto expects timestamps to be CLOCK_BOOTTIME which is nanoseconds
since boot including suspend. This discrepancy is unfortunately hard to
catch on test devices which are plugged in and don't suspend.

Bug: 400774865
Flag: com.android.server.power.feature.flags.enable_app_wakelock_data_source
Test: atest PowerServiceTests
Change-Id: I1b0876ee41ab5e9ef57180736c61dd4cfe651605