Phone: Display toast message for blank USSD request

As per spec 24.080, ussd response string should be
between 1 - 160.
When user requests SEND_USSD with blank/no
input, display toast message prompting user
to enter valid input.

Bug: 7471891
Change-Id: Ic7b7b89e8a683011048a19cabd1d36a27bbae953
Default back to LTE/WCDMA/GSM if supported

Currently after unchecking the 'Use only 2G networks' option, the
default Network Mode is 'WCDMA Preferred'.  This patch will change
the behaviour so that if the mobiel supports LTE, the default mode
upon unchecking 'Use only 2G networks' is LTE/WCDMA/GSM

Change-Id: I80be0eee564aa76e5cd978595bff98eb461e00f7