Extreme battery saver: Cap max cpu frequency.

- Lower the max CPU frequency on battery saver.
- We have different sets of frequencies for screen-on and screen-off.
- The frequency configuration is from R.string.config_batterySaverDeviceSpecificConfig.

Bug: 68769804
Test: Manual test with settings put global battery_saver_device_specific_constants cpufreq-n=0:1324800/4:1497600,cpufreq-i=0:1900800/4:1958400
  using cat_repeat /sys/devices/system/cpu/cpu{0,4}/cpufreq/{cpuinfo_cur,scaling_max}_freq

Test: atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/CpuFrequenciesTest.java
Test: atest $ANDROID_BUILD_TOP/frameworks/base/services/tests/servicestests/src/com/android/server/power/batterysaver/FileUpdaterTest.java
Change-Id: Ie1bb11c46c0d4730129fe9d82e86d8acebe0b560