Fix App battery usage crash

userContext.getApplicationContext() returns null, which cause crash.

Use context instead to fix.

userContext was used to support multiple profiles, since the app's user
id is also passed to getAppBatteryUsageData(), userContext is not need.

Fix: 269545838
Fix: 275957358
Test: Manually check the battery usage is still correct if an app is
      installed on two profiles
Change-Id: I41a80e4c6edc338affb6eec0cf12da3c438cd7fd