LogEvent: Avoid unaligned assignment of value

Since we can have 'T' be things like int64_t, and we're arbitrarily
incrementing 'mBuf', we're not always assured 'mBuf' will be
properly aligned for an assignment.  We do a check and use
memcpy() if it's going to be unsafe.

Test: TreeHugger
Bug: 148549384
Change-Id: I76fd12a3241a997acf4625c9ce1176ae76c5bff2