Drop mIsSystemAuthority.

This field is set to true iff mSystemEntity is not null, so it is
redundant. The same is true for the XML representation.

Bug: 336315393
Test: TH
Flag: android.app.admin.flags.dont_write_is_system_authority
Change-Id: I2feb327bf4709f37ccccc23a98daf09c7c33f387