Replace OS_ANDROID by __ANDROID__

__ANDROID__ is already set by the build environment
so we don't need to roll our own OS_ANDROID

Everything is replaced except system/btif/src/bluetooth.cc
as it's not present in internal branches and will be
removed in AOSP as well when b/280533897 will be fixed

Test: TreeHugger
Bug: 279502784
Change-Id: Ic2f72ae52f101819a12f0078cf1dc28c622efb1a