audio/AVRCP: Always assign a player to TG role

Currently the code does not assign a player if version is bellow 1.3,
but sometimes the version may not be initialized correctly e.g: record
could not be fetch.

Futhermore by assigning a player for 1.0 CT it won't break backward
compatibility as no commands will be sent but adds the possibility to
respond properly in case the CT is sending commands above its own
version which is a violation of the spec but unfortunately there exists
such stacks on the market.
android: Add README file with instructions

This file cotains help on how BlueZ for Android should be build, run
and test. Some hints and examples on how BlueZ can be intergrated into
Android are present as well.
android/client: Fix crash in tab completion

When user pressed tab on third or later argument when
first two did not make sense tool crashed.
Now method is checked for NULL before accessing help field.
android: Add common place for adding compiler flags in Android.mk

This will contain CFLAGS common for all BlueZ code.
android: Suppress compiler warnings not used in autotools build

Android build system is enabling some additional warnings that are not
enabled when building with autotools. This avoids spurious warnings
while building on Android.
android/hal: Add support for start/cancel device discovery commands
android/hal: Add support for handling discovery state change event
android/hal: Extract bt property repacking code for further reuse

This extracted part of code can be reused for repacking remote device
properties.
android/hal: Add device found event handler

This is called when new remote device is found.
android/hal: Add device state changed event handler

This is used to report property change of already reported remote
device.
android: Add missing discovery state definitions to IPC header
tools/mgmt-tester: Fix calling test_setup twice
tools/mgmt-tester: Fix sending right set_discoverable parameters in setup
tools/mgmt-tester: Add basic BR/EDR limited discoverable on test cases
tools/mgmt-tester: Add basic LE-only limited discoverable on test case
tools/mgmt-tester: Add basic LE-only discoverable on test case
tools/mgmt-tester: Add LE test for connectable off when discoverable
tools/mgmt-tester: Add LE test for set_connectable when limited discoverable

We also need to update the order of the initial settings since limited
discoverable is only allowed when powered on.
tools/mgmt-tester: Rename 'LE-only' to 'LE' for nicer logs
android: Fix compiler flags with Android version

This fixes commit that removed PLATFORM_SDK_VERSION when building
haltest. Now all android targets get this define.