ThermalManager: Don't hold mSamples lock while calling power HAL

Summary: There is no need to hold the lock while making the call to the power
HAL. In the case the HAL call takes a long time due to hardware reads,
holding the lock may cause lock contention from other threads.

Test: build passes

Tags:
Change-Id: I6b1f8d1228de17e4bf188bd99037b02ac7ebff1a
Signed-off-by: Abhishek Gadewar <abhishekgadewar@meta.com>