vc: Simplify the connection logic

Removes the ambiguous first_connection flag which was partially
the device_ready flag and partially connecting_actively flag.

Bug: 254810994
Tag: #feature
Test: atest --host bluetooth_vc_test --no-bazel-mode
Test: atest BluetoothInstrumentationTests
Change-Id: I55015c0cacf0badbf584bf27ed9e189673cb47f6