Frameworks: Add warning to SystemProperties.get

Add a warning that calling get() is unsafe when the system property
may not contain a valid UTF-encoded string.

(cherry picked from commit 6bf1cff1)

Bug: 63177684
Test: m
Merged-In: I68e3096d770a32fe204be5c130a3ada9d3db7c0b
Change-Id: I68e3096d770a32fe204be5c130a3ada9d3db7c0b