Add GnssAutomaticGainControl to GnssMeasurementsEvent (frameworks/base)

AgcLevelDb has been a subfield of GnssMeasurement. However,
GnssMeasurement is only reported by the chipset when there
is GNSS signal. For example, in deep indoor, or when there
is strong jamming signal, no GnssMeasurement can be reported,
and thus no AGC value can be reported.

To resolve that, we are adding this GnssAgc to GnssMeasurementsEvent
directly, so that the client can get AGC values even without GNSS
signal (i.e., without GnssMeasurement).

Bug: 206670536
Test: atest GnssAutomaticGainControlTest
      atest GnssMeasurementsEventTest
Change-Id: I97c7ec98b6e8cc977c6e879b52a8cf75c355ae33