AAPT2: Set array element keys

Many google3 apps using lemon resource filtering had difficulty
converting to aapt2. AAPT2 never set the name of the ResTable_map name
value mapping.

Bug: 78472540
Test: blaze build java/com/google/android/apps/gmail:Gmail_go_release
and checking that all array resources are present
Change-Id: I3d253a4c52c6100085e4c1083f4c36a943f11c76