Use getpwnam()/getgrnam() instead of AID_* macros (dumpstate)

AID_* values are defined in libcutils private headers and hence
is not exported. Use getpwnam() instead to get these values.

Test: device boots
Test: bugreport
Test: dumpstate_test
Bug: 63135587
Change-Id: Ib35fe4fc4524eb1885616a9a1403ceb09f713777