[Mobile] Use CellSignalStrength.getNumSignalStrengthLevels

The existing mobile pipeline was mistakenly using a const val `4` for
the default number of levels. It should have been defined as `5`,
representing the levels 0-4 discretely.

This change configures the MobileConnectionRepository DEFAULT_NUM_LEVELS
to be defined in the same way as the old MobileSignalController.

Test: tests in pipeline/mobile
Test: manual via demo mode:
Test: adb shell am broadcast -a com.android.systemui.demo -e command network -e mobile show -e inflate true -e level 1
Fixes: 299689440
Flag: NONE
Change-Id: I26e52afdc09f126a9aba8d89af819c84e020fa1c
(cherry picked from commit 963a9fc8)