New Autofill field: FIELD_AUTOFILL_SESSION_ID

This field contains the autofill session id (Thank You, Captain Obvious!), which
is useful to correlate autofill metrics that are not logged on
AUTOFILL_REQUEST.

Test: atest CtsAutoFillServiceTestCases # to make sure it didn't break anything
Test: manually run some test and watched
      'adb shell logcat -b events | grep sysui'. Example:
     [757,913,758,4,806,android.autofillservice.cts,871,android.autofillservice.cts.LoginActivity,914,2,915,2,1456,-2045861404

Fixes: 80093094

Merged-In: If4b1c9ab616b2ba580745096894997eb7edb3e34
Change-Id: If4b1c9ab616b2ba580745096894997eb7edb3e34