fastboot: clean up CheckRequirements

CheckRequirements() had various issues that are cleaned up here,

1) Move from C string parsing to C++
2) Moved from C data structures to C++, including fixing memory leaks.
3) Removed the 'cur_product' global and the 'query_save' function that
   stores it
4) Actually writing tests for the parsing function for
android-info.txt
5) Check that a variable needs to be checked for a given product before
   trying to read it.  Previously, fastboot would fail if a variable
   isn't recognized on a device, even if the check should be ignored.

A lot of flexibility is allowed for the input strings, to keep
backwards compatibility with the previous parsers.

Test: fastboot works, unit tests

Change-Id: Idc3bba8b8fe829d8eefe5f6c495e63a9441c0b60