Implement new BS warning / notification flow (1/2)

- Show the battery saver confirmation dialog only for the first time.
- Start counting # of manual activations, which will be used in a
follow-up CL.

Bug: 74120126
Test: Manual tests with ./vendor/google_experimental/users/omakoto/android-battery-tester
Test: m -j ROBOTEST_FILTER=BatterySaverUtilsTest RunSettingsLibRoboTests
Test: cd frameworks/base/packages/SystemUI/tests && \
        atest src/com/android/systemui/power/PowerUITest.java src/com/android/systemui/power/PowerNotificationWarningsTest.java
Change-Id: If6a081a6222e6a87c4cd332364c89856e7648a36