Generate Compdb file in the output directory

This change modifies the location of generated compdb file from
$ANDROID_BUILD_TOP to where compdbOtuputProjectsDirectory points to,
under $OUT_DIR.  This is especially required for the build environment
that maps $ANDROID_BUILD_TOP as read-only, such as Gitc.

Test: m SOONG_GEN_COMPDB=1 nothing
      examine generated compile_commands.json
Signed-off-by: Changyeon Jo <changyeon@google.com>

Change-Id: I86700f89b613a443a33982ecd5501b0f68a62be7