SignalStrength: Hotfix TD-SCDMA

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
Set empty string to the property if value is null

Issue is if the received value is null, then while
preparing complete property value for MSIM the null
value is converting to "null" string. Fix the issue
by adding null check to the received value.

Change-Id: Ib7a8ea5ce5b1341df6155ec34c25a016eb4ec667
CRs-Fixed: 622695
Validity Period support for MO SMS in 3gpp

Add Telephony framework support for Validity Period in 3gpp
for MO SMS

CRs-Fixed: 627981

Conflicts:

	telephony/java/com/android/internal/telephony/ISms.aidl

Change-Id: Iee7358ca1971441f0567e428b587562294fe29bb
Add support for default data subscription setting.

Default data subscription setting holds the default preference for data
subscription. It will be updated when user changes data preference,
sub deactivated or card removal.

Current set preferred data subscription API does not update the default
data subscription preference. This change is to handle temporary DDS switch
by apps like MMS.

New APIs to update and retrieve the default data subscription are added.

Change-Id: I495266cff2a5319c57279d9decb77d6a87bbfae8
CRs-Fixed: 642210
SignalStrength: Hotfix TD-SCDMA take 2

The previous approach worked for 3G, but failed with TD-LTE. Let's
try something different to make them both work, and use the derpy
LTE checks only if a GSM signal is present.

Change-Id: I77e3b5a1e0f99df30c32b102918af418a20077c1