ArrayUtils: add concat() for byte arrays

Concatenating byte arrays is a common thing that needs to be done,
especially in the locksettings package, so add a utility method for it.

Test: atest com.android.internal.util.ArrayUtilsTest
Merged-In: I099c83155456139729f4d73f5873a172590ee7e9
Change-Id: I099c83155456139729f4d73f5873a172590ee7e9
(cherry picked from commit 4dd9540f)