Add tool to help gathering fieldnull data

The libfieldnull.so tool allows one to determine how many of a classes
instances have various fields set to null. This can be useful but
actually gathering the data for a large number of processes can be
difficult. This adds a check-null-fields.py program that will attach
the fieldnull agent to every process it can on a device and record the
results.

Test: ./tools/field-null-percent/check-null-fields.py 'Ljava/lang/Class;.name:Ljava/lang/String;'
Bug: 32635074

Change-Id: I308387a0a4e9366020cb5e18c5cb5a70d7d6ddb0