cherry-pick from oc-dev: docs: Note regarding device IDs, Build.java

Added note within getSerial() in android.os.Build class. Device
identifiers, such as hardware serial numbers, can be modified if a
device is rooted. Others can use key attestation to discover these
changes. Added a note to this effect within this method.

Test: make ds-docs -j8

Bug: 38312326
Change-Id: I76e3f6a15b9a8673ca3304218653e8cd26da38d7
Original-Change-Id: Ibd2dab621c14683e12a213957b567ee49b8d00fd