Lock BatteryUsageStats snapshot directory

From documentation:
"File locks are held on behalf of the entire Java virtual machine.
They are not suitable for controlling access to a file by multiple threads within the same virtual machine."

This means that we should use two locks: one internal to system server,
and another cross-process.

Bug: 216454829
Test: atest FrameworksCoreTests:BatteryStatsTests
Change-Id: I7689ab91a0759f99da700383647f4f8a1fae4362