ArrayUtils: rename concatElements() to concat()

concatElements() was probably named as it is to be like addElement() and
removeElement().  However, that doesn't really make sense because the
parameters of concatElements() are only arrays, not array elements.

Rename it to concat(), as that is just as clear and follows the example
of Guava's ObjectArrays.concat().  And I'd like to add a variant that
operates on byte arrays, and would like to call it concat().

Also clean up the tests a bit.

Test: atest com.android.internal.util.ArrayUtilsTest
BYPASS_INCLUSIVE_LANGUAGE_REASON=existing code in SettingsBackupAgent
Change-Id: I21e958d3d7b3366ec724e757ac4ccc50b36abee1