General tidy up of android.system

This commit changes StructIcmpHdr to the byte[] factory that it
effectively already is. StructIcmpHdr is not referenced by anything in
android.system.Os so is not the same as other Struct classes.

It also add toString() to structs / containers where missing and makes
a field final.

Bug: 135660232
Test: build / boot
Test: atest CtsLibcoreTestCases:libcore.android.system.OsTest
Change-Id: I84985c817234aadc2380a08a6afa71400366a6f6
7 files changed