BatteryControllerTests: create TestableContext as a @Rule

This is the way that TestableContext's JavaDoc recommends, to let it
clean up after itself once the test finishes, so we should do it anyway.
However, it won't fix b/339924248 on its own.

Bug: 339924248
Test: $ atest InputTests:com.android.server.input.BatteryControllerTests
Flag: TEST_ONLY
Change-Id: I53b40671286530a7c411cfff6bc08239f5baf13b