SignalStrength: Hotfix TD-SCDMA 1 + 2

These checks need to be reviewed, they're making unwarranted assumptions
about signal value formats and possible tech combos.
This appears to work for all possible scenarios I was able to test,
but the "LTE checks" shouln't be necessary at all.

Change-Id: I889e17e79ca2b1b5fcaa393741e19bef9d3f4229
Missing break in switch statement

Change-Id: I462c0c5fec96a850032266b04419ec409ad02174
Use a content observer for getting the default APN

Instead of executing a query of the database for the
default APN on every network state change, an observer
is used on the database to be notifited when the apn
has changed. This prevents unneccesary queries on the
the database.

CRs-fixed: 662137

Change-Id: I3155bd8b53a1877413ed54a861df1bfb1853b9e4