Improved notification interruptiveness calculation.

Apparently comparing Spannables is dangerous because
the various Span classes do not implement .equals() in any
meaningful way, so all CharSequences must be converted to
flat Strings before being compared.

Lots of additional debug code remains, for the next time we
don't understand why an innocuous notification update
appears to be interruptive.

Test: atest com.android.server.notification.NotificationManagerServiceTest
      atest com.android.server.notification.NotificationTest
Bug: 78643290
Change-Id: I1c282238687f28b5b197e28a4b878dc697049f4d