Actually store category set via JobInfo#setCategory.

When JobInfo#setCategory was added, the value passed in was
never actually stored. This change adds storing and validation
of the provided value.

Bug: 419047126
Test: atest ./tests/JobScheduler/src/android/jobscheduler/cts/JobCategoryTest.java
Flag: android.app.job.job_category_apis
Change-Id: I6083732ee4d41fefdb459494ea882d183f1dec54