Rudimentary automated testing for LE beacons.

Talking with the Bluetooth team, we know there are more robust
testing strategies coming with Root Canal and Blueberry, but those
aren't ready for use today.

As part of building out the "Nearby devices" permission we'd really
like to get automated CTS testing in place, so this change adds a
very rudimentary testing mode to the Bluetooth stack.  When enabled,
this mode suppresses scan results returned from the native stack,
and replaces them with a set of hard-coded BT LE beacons which can
be used for CTS purposes.

This new testing mode can be controlled via the commands below which
are only available to AID_SHELL, and which CTS can invoke using
executeShellCommand().

$ adb shell dumpsys activity service \
    com.android.bluetooth/.btservice.AdapterService \
    set-test-mode [enabled|disabled]

Bug: 179808874
Test: atest BluetoothInstrumentationTests
Tag: #feature
Change-Id: Iddae5dc450dd83071bdb62841d28667dddc48474