[MSKP] update Android MSKP capture documentation

Change-Id: I8294f76ac91bd2fc303b3c9e5e012858740008e8
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/969216
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>