DexDump: Leak in FieldMethodInfo.signature

Free pMethInfo->signature when filled by getMethodInfo.

Change-Id: Id278d732374daceb7b8b05be7f433af25eee0c27
Allow max free size of heap to be configured.

This allows the max free to be larger on devices with more memory and
thereby avoid GCs (that may be more frequent due to larger bitmaps).

b/6606849

Change-Id: I4063a303ea4be3c1f227259d699fb238a806451f