Build support and abstractions for Linux build

This has a large number of fixes to build on Linux/ChromeOS. In general,
it is making sure the BUILD.gn and Android.bp for a directory structure
match and it has all the configs + deps needed to build.

Besides a number of simple fixups (missing headers), there are also
a few abstractions that need to be noted:

* All of audio HAL is stubbed out (using the host implementation)
* Parameter provider and system_properties (under gd/os) now have
  a linux variant that changes where configuration files are kept. The
  current location for linux is now /etc/systembt. This implementation
  was copied from the host versions (which seems to be used for host
  testing).

Bug: 176847256
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I1a6b3b362cedbe5f675794a115a330cc7f85f9c1