ANDROID: kbuild: Add support for installing out-of-tree kernel headers

Add support for installing out-of-tree kernel headers. The
"headers_install" target was copied to the external module compilation
section (lines 1245:1270), and the src/obj variables were tweaked to
include $(KBUILD_EXTMODULE).

Bug: 403385861
Bug: 162332589
Change-Id: I26270922bb90a0f446dfdc0fab2c5d90ef6e32f3
Signed-off-by: Elliot Berman <eberman@codeaurora.org>
[willmcvicker: make easier for merges by adding ext mod section]
Signed-off-by: Will McVicker <willmcvicker@google.com>