Update AOSP tracking tag to 4.4.4_r2

This commit switches the tracked repositories to use 4.4.4_r2
instead of the older 4.4.3_r1.1 that was used before. The prebuilt
kernel does not need to be updated as the AOSP/upstream repository
has received no changes between these tags (they point to the same
commit, that is).

Change-Id: I3cc8ebb9f61533d5b6c22c0b00de20ae8dc8969d