Extreme battery saver: Tweak to file saver

- Restore the default values after a runtime restart.
- debug.batterysaver.no_write_files sysprop to avoid writing max_freq

Bug 63527520
Test: manual
Test: atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/FileUpdaterTest.java

Change-Id: Ic4783f181252b063cfe7ea6fee47b9241f1688cc