Don't call into ActivityManagerInternal outside system process.

If ActivityManagerNative.isSystemReady is being called in a non-system
process, just return true instead of calling into
ActivityManagerInternal since system should be ready by now and
as ActivityManagerInternal is only available for system services.

Fixes: 32700143
Test: Test referenced in the bug is passing.
      adb shell am instrument -e class com.android.androidbvt.SysUIGSATests -w com.android.androidbvt/android.support.test.runner.AndroidJUnitRunner
Change-Id: Ibb594f887f7f5391c34c686f2667a6696e4268a2