BootReceiver: cap the number of sent kernel memory corruptions

Save the bandwidth by sending only the first few reports from a running
kernel.

Also fix a bug in the old code (strings must be compared with equals()).

Bug: 172316664
Bug: 182159975
Test: running KFENCE manually
Signed-off-by: Alexander Potapenko <glider@google.com>
Change-Id: I3eb13596b5d9006637e110d000f64c46661cac4d