Make BatteryDatabaseManager singleton

In BatteryTipLoader, two threads may access BatteryDatabaseManager
simultaneously. In this case thread A may close the database thread B
holds, then settings will crash.

In this cl, we make the BatteryDatabaseManager as singleton and
synchronize all the database related method. Then it shouldn't have
the crash anymore.

Bug: 73346734
Test: RunSettingsRoboTests
Change-Id: Ib53b2894b25155cca0c6ec60d1a816663d27a578