Dump 'external fragmentation unusable index'.

Currently, /d/extfrag/unusable_index is read by dumpstate to print the
unusable index for each allocation order. From Android R, platform code
is no longer allowed to access debugfs since its ABI is not considered
to be stable. Hence, instead of reading /d/extrag/unusable_index, this
CL calculates unusable index information from /proc/buddyinfo instead.
Here are the calculated values and those read from
/d/extfrag/unusable_index for comparison.

------ FRAGMENTATION INFO (/d/extfrag/unusable_index) ------                                        
Node 0, zone      DMA 0.000 0.399 0.720 0.884 0.966 0.978 0.982 0.992 1.000 1.000 1.000            
Node 0, zone   Normal 0.000 0.101 0.444 0.561 0.680 0.780 0.906 1.000 1.000 1.000 1.000            
                                                                                                   
------ EXTERNAL FRAGMENTATION INFO ------                                                          
Node 0, zone      DMA 0.000 0.399 0.721 0.884 0.966 0.979 0.983 0.992 1.000 1.000 1.000            
Node 0, zone   Normal 0.000 0.101 0.445 0.561 0.681 0.781 0.907 1.000 1.000 1.000 1.000  

Bug: 134669095
Test: adb shell dumpstate

Change-Id: I3e84505cca8ef2327ad7058385d3f60928b2e436