Perf: improve updateGlobalCellId

String.format() used in updateGlobalCellId is heavy and slow. Unit
benchmark shows that formatSimple is about 5-10x faster.

Bug: 168639120
Test: Confirmed the calculated IDs are the same. The dogfood phone works
with master local build.

Change-Id: I7707a42efc74eadb133100d8b2b769301e56c100
Signed-off-by: Li Li <dualli@google.com>