Support CSV hiddenapi flags in class2greylist

Access flags for hiddenapi are being converted from one file per flag
to a single CSV file in the format:

  <api signature>,<flag1>,...,<flagN>

Adjust `class2greylist` to produce such a CSV instead of text files.

Test: m, phone boots
Test: atest art/tools/class2greylist/test
Change-Id: I6592027256e42d87589512ada1a8ce4bd1c03e63