TelephonyTester: simulate signal strength level for automation

The change allows simulation of signal strength level change through adb command, like voice_reg/data_reg etc.

Although the function has been provided by hidden menu, adb command approach can make automatic test much easier and robuster.

Like all other simulations, it is only available in debuggable builds and doesn't impact product builds.

Bug: 256874469
Test: Manual test for API used via adb shell
Flag: EXEMPT(test only)
Change-Id: I6f882e95f62000437d2ac6cb631dad5a68992565

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Validate input signal strength

The input signal strength can be set from 0 to 4, matching the enum values of SignalStrength.
Make TelephonyTester validate input to only permit valid values.