Undefine NAN after including it in Looper.h

Looper.h needs to include the header for unordered_map, which itself
includes math.h, which defines the macro NAN. Some HALs use enums called
NAN, which causes a build error if the NAN macro is defined. We need to
undef NAN in these cases after including Looper.h.

Bug: 195020232
Test: presubmit: checkbuild
Merged-In: I63bba8ea809a12571ddc88cd1d20f2adeedf0b30
Change-Id: I63bba8ea809a12571ddc88cd1d20f2adeedf0b30