Improvements on dumpsys activity broadcasts.

- Modifies ReceiverList to dump the number of receivers / app
- Adds 'receivers' argument, so it shows only the registered receivers (similar
  to the 'history' argument )
- Adds an extra --uid argument to 'receivers'

Test: adb shell dumpsys activity broadcasts receivers
Test: adb shell dumpsys activity broadcasts receivers --uid 1000

Fixes: 226224551
Change-Id: Ib191c0d3309057e91f0421e3dc6dc9f1a41b14ac