Fixed power manager CTS test.

Added an annotation necessary to allow the power manager CTS test to
access the non-public method nap.

Test: adb install -r \
      out/host/linux-x86/cts/android-cts/testcases/CtsOsTestCases.apk
      adb shell am instrument -e class android.os.cts.PowerManagerTest \
      -w android.os.cts/android.support.test.runner.AndroidJUnitRunner

Change-Id: I91e591be608f5040bc035a203d7e94c83d50bf15
Fixes: 77907164